## Transponder, radio and PilotAware antenna fitting in a non-metallic skinned aircraft

There is often talk around the hangers about poor radio signals and how some aircraft can be heard from great distances while other fail to make contact with a ground station even nearby. One of the issues often overlooked is a missing or wrongly sizes ground plane. There are many myths about the purpose of a ground plane and I hope in this short article to give you guidance on the correct size and purpose of a ground plane.

Before we look at the ground plane lets set expectation on maximum distance you can be heard.

The civil air band radio is in the VHF (Very High Frequency) band and under normal weather / tropospheric conditions, it requires radio line of sight between the aircraft and the ground station; thus, the maximum range is the distances to the radio horizon from the aircraft to the ground station, for simplification we are assuming the ground station is at sea level.

The radio horizon distance is given by the formula: D= K√h

Where: D = distance in nautical miles (NM), h = height of aircraft’s antenna above ground level and K = 1.23 when h is expressed in feet

So, when we are flying at 2,000ft the maximum range to a sea level ground station is 55.00NM or at 3,000ft 67.34NM

The same is true for transponders, CAP1391 devices e.g. SkyEcho and PilotAware transmissions, however the limited power and antenna arrangement of CAP1391 and PilotAware means the range is less.

#### What is a ground plane is and why we need one?

Keeping this simple, it provides a ¼ wave monopole antenna (also know as a Marconi) with its required counterpart allowing the flow of the electrical current which generates the electrical and magnetic waves that make up the RF signal. The earth acts as a type of electrical “mirror,” effectively providing the other quarter wavelength making it equivalent to a vertical dipole. This in turn helps present the correct impedance to the transmitter, thus allowing the maximum power transfer into the antenna, which results in the most efficient transmission of the signal. Most antennas sold for Microlights and light aircraft are ¼ wave.

##### Does size matter?

I’m afraid the answer is yes it does! Bigger is generally better, but again there are exceptions and a minimum size for the efficient working. The minimum size is a radius of ¼ the wave length of the signal being transmitted.  In basic RF theory we were taught that bigger is better, although the benefits of size are less important above 3 times the wave length. In aviation while we are flying along we want our signal to be picked up both above and below the aircraft, the larger the ground plane the bigger the blind spot we will have on the underside of the ground plane.

##### Is the shape important?

No, a ground planes can be square, circular or many different shapes. Say you had a metal aeroplane, the whole fuselage is the ground plane for your radio antenna, and your transponder antenna. So long as you have at least ¼ wave length in all directions, then the shape doesn’t matter too much. However as we want the lightest ground plane we can get, a circle is the optimum shape and supports equal propagation of the signal in all directions.

##### What should the thickness of the ground plane be?

Any, you can use foil tape or discs of different thicknesses so long as it is mechanically robust enough and conductive.

##### Can it be a mesh or does it have to be solid?

It can be a mesh (the tighter the mesh the better) or even multiple radial wires, typically the more the better 8 or 16, but for our purposes a solid circle is normally the easiest and best option.

##### What should it be made of?

Copper, however it can be any material which is a good conductor. Aluminium is a good conductor and can be used, however you need to be mindful of the galvanic effect between a stainless-steel antenna and the ground plane. This may lead to connection issues in the future, thus I would recommend a thin copper disc.

##### Why do some avionics companies state to use a square ground plane?

Well I assume this is just convenience, so long as the diameter of the circle below can fit within the square it will work fine. Just make sure the antenna is in the middle!

##### What does all of this mean in practice?

Minimum diameter of ground planes in aviation should be:

N.B. Always round the ground plane size up not down.

Transponder: frequency 1090MHz, Wave length 275mm, ¼ wave length 68.75mm

Minimum ground plane diameter 137.5mm

Radio: lowest frequency 108MHz, Wave length 2775.85mm, ¼ wave length 693.96mm

Minimum ground plane diameter 1,387.9mm

PilotAware: external antenna fitting, frequency 869.5MHz, Wave length 344.79mm, ¼ wave length 86.19mm

Minimum ground plane diameter 172.39mm

##### Antenna Theory Is Complicated

Antenna theory is complicated, but if you have a ground plane with the correct minimum size and the antenna mounted in the middle this will be one less problem.  But let’s quickly prove this is fact and not just some text book exercise. As an extreme lets consider an antenna without a ground plane vs the same antenna with a correctly sized ground plane.

An external PilotAware antenna without a ground plane is put under test using a low cost VNA (Vector Network Analyser)

Lorem ipsum dolor sit amet, consectetur adipiscing elit. Ut elit tellus, luctus nec ullamcorper mattis, pulvinar dapibus leo.

As can be seen opposite the resonant frequency of this antenna is not 869.5MHz as it should be! It is showing a resonant of over 1GHz. The impedance and VSWR for the antenna was also way out, in fact the reflections from the imbalance could damage the transmitter or at best the radiated signal will not be optimal.

Let’s add a make shift ground plane out of kitchen foil of the recommended size above.

opposite you can see the effect of adding the ground plane. The resonant frequency has shifted down to be close to the 869.5MHZ that we are looking for, the VSWR was down to acceptable limits and the impedance was close to 50 ohms.

Hopefully this graphically demonstrates the importance of having a correctly sized ground plane!

One last Fact! The monopole antenna was invented in 1895 by radio pioneer Guglielmo Marconi

## This is a reflection of how I went about changing the propeller, it is not meant as a "how to" and no liability will be taken form any issue others may have by following this post!

We made the decision to change the original Neuform Propeller after noticing some hairline cracks across the back of the prop. This together with an accumulation of small chips meant  it was highly unlikely to pass a factory inspection, thus the prudent thing to do was to change the prop.

Neuform Props are very good, but expensive props, so we chose to replace it with a Kiev which at the time of writing was around £1,000 less.

The first stage is to remove the top cowling, we also later removed the bottom one too, to allow us to have a good look around at the same time. This proved to be a prudent thing to do as we found our radiator bracket was broken too!

Next remove the spinner, a tip here which some may not know, is to hold the allen key bolt and undo the nyloc nut with the spanner, else you may start to round the allen key head.

NB: I found out the hard way, that you should mark the back of the spinner plate and the flange to ensure the spinner goes back on in the correct position!

Now we need  to remove the old prop by undoing the six M8 retaining bolts. These are the long ones which screw through into the flange.

After these are removed the propeller will come off still clamped in the hub.

The prop with its adaptor plate is now removed.

## Removing the the lugs

Now for the  “fun” part, removing the lugs from the flange.

The lugs are used by both Neuform and Warp drive props, but not by Kiev props.

These lugs are press fit and difficult to remove. First spray them with AC50 and then  have a cup of tea while this goes to work!

The best way we found to remove them safely  is to use an M8 bolt through a 17mm socket, this acts a puller and pulls the lugs back through the flange plate without damage or shocking the gearbox.

Once they are almost through they will easily tap out.,

## Fitting the new prop

First lay the prop out and loosely clamp the prop blades in the hub. Ensure the key marks are aligned (see image) this is just tight enough to hold it all together, while we loosely bolt the prop on to the flange using the new adaptor. Note the new adaptor plate can only be purchased from TLAC  and was around £120.00 inc VAT.

The bolts to hold the prop on are 110mm long if you purchase the correct ones. However  using these bolts mean they have 25mm of thread and about 12mm of this thread is  within the flange plate holes. This is not good engineering practice and after checking with the BMAA they agreed. The solution is to buy the next size up (120mm) and cut them down by 10mm thus giving 15mm of thread which means you will have about 3 threads out of the Nyloc and much less thread within the flange.

The bolts we used are M8 grade 8.8, DIN 931 120mm, cut to 110mm.

Once the prop is mounted and all the bolts are in, but not tight, they should be loose enough to allow the blades to be turned within the hub, but tight enough to stop them moving to easily.

Now the pitch can be set. We have found mounting the the pitch tool 48.5cm in from the tip on the inside of the mark and setting a pitch of 25 degrees gives the correct revs, 4,800 rpm on the ground. this TLAC confirm to be the correct test that the prop is pitched correctly.

Work your way around all the blades in turn setting the pitch.

Carefully do the bolts up working your way around the opposite  bolts that clamp the blades first.  These should be be done up to a torque of 15Nm. Now before doing up the main bolts recheck, loosen and adjust and re tighten as needed the pitch for each blade.

Lastly do up the mounting bolts that go through the flange, again do this progressively working across the opposites, these are torqued up to 25Nm,  then once again check the pitch and adjust the pitch if needed.

All that is left is to ground run the aircraft. Check your static full power rpm is 4,800 rpm +/- 50 rpm, our brakes don’t hold at this power so we check that we get this during the takeoff.

The torque of all the bolts need to be checked at 1hr, 25hrs and 50hrs after fitting the new prop.

I hope this has will help others, even if it doesn’t it will serve as reminder to myself if I have to do a third prop conversion!

Lastly you will need a BMAA inspector to sign your change of information form.

## The little things count too when doing a preflight check!

Today we planned to fly to Le Touquet, but the weather had other ideas! As we could not fly to Le Touquet we decided to try Sandown, Isle Of Wight.

We started by cancelling the flight plans filled via Skydemon and then we set off. Initially, the cloud base was around 800ft but, soon and as indicated, this opened up and we climbed to 2,000ft and then 3,000ft as it continued to improve.

The flight down apart from the claggy start was largely uneventful. We had been having issues with our Funke TRT800 transponder, it is intermittently not being seen by ATC and locking up, however on the flight down it behaved impeccably. The only issue we had on the way down was the landing, I touched down gently and then hit a bump and a gust of wind which saw us back in the air, but not by much. Next, another gust just as we were touching down and 90 degrees to the runway saw our right wing lift. I caught this on the stick and controlled it with the aileron. I was wondering if I could have done better as I watched Colin land behind us in his C42 and saw him have the same issue, it was like watching an action replay however, this time the gust had taken him far to the left of the runway.

## The journey home, however, was much more eventful!

First, the Transponder didn’t seem to be working, so we called Fanbourgh Information and asked for a transponder check, we tried Ident and they could not locate us, we tried another squawk but, we could not change it, so we need to send it back to Funke for a service repair.

About 40 mins from Chatteris we heard a loud ripping sound, it was like someone ripping velcro apart in our ear! followed by a humming sound! we looked at each other in disbelief, the aircraft was still flying and the controls responding normally. The humming stopped and all seemed normal so we decided to continue, and then another loud ripping sound more humming and heavy vibration through the stick and pedals!

What could this be? what should we do?

We looked for an alternative and near airfield, our best option was Old Warden so we head towards it, then as suddenly as it started, it stopped, we continued towards Old Warden with no further occurrence of the issues.

What could it be? Our best guess was that the velcro fastening between the wing and the aileron may have separated and the noise and vibration were due to the resulting airflow between them.

On to Chatteris and 10 miles out we heard Skydive One (the parachute plane) called it was taking off, wanting to get straight in we called Skydive one and advised them of our situation and they delayed the drop until we had landed. On final we put the first stage of the flap on, as the aircraft’s pitch changed the vibration and humming returned, we decided not to put the second stage on. After landing we inspected the aircraft looking closely at the ailerons and the skins, nothing! Then we spotted the windscreen tape had lifted in the middle of the vertical section, could this have been the issue? it would make sense re the ripping sound, but what about the humming and vibrations? we reasoned that the humming could also be due to this and possible the vibrations too if it was disturbing the airflow. The tape was removed and we took her up for a test flight and to our surprise, the issue was solved!

This reminded me of Rob Mott’s article in Microlight flying magazine where he talks about knowing your aircraft and not to panic if you have an issue. We knew the aircraft was still flying, we knew it was responding normally to all control inputs so we minimised the issue by flying conservatively and landing at Chatteris as soon as possible.

Little things like the edge of the tape lifting are important to check on a pre-flight check as well as the major items!.

## Should we have performed a precautionary landing?

We could have put her down in a field or at Old Warden, but she seemed to be flying OK and what damage might have been done during the landing? Old Warden is PPR and was NOTAM’d for model flying.

There has been some discussion if we were right to have continued flying or not. What is your view?

## ROCC – AGCS training is FAB

This weekend my friend Simon and I attended  ROCC (Radio Operators Certificate of Competence) training which following a Written and practical exam and approximately 2 days on the job training enables you to operate an Air-Ground Radio station. I don’t have a great desire to provide this service from where I fly from, but I thought it sounded interesting and would serve as a useful RT refresher. When I first saw it advertised on a Facebook Group (see sometimes good things come from FB!) the chap advertising it was offering a familiarisation of ATC including a tour of Humberside’s ATC unit. I responded to Lee Stephenson’s ad and found him to be professional, communicative and helpful the course was also offered at what I thought was a good rate, £150.00 including the exams, so I booked up two places. Lee operates as LAS Aviation Services.

As I had been teaching during the day Simon kindly offered to drive us up to Humberside airport on Saturday evening; Lee had kindly included car parking in the training cost.

On arrival, we were both hungry and so we went from the car park to the hotel (Hampton by Hilton) stopping only to look at Thunderbird 3, yes you read that correctly! Eastern Airways code is T3, so at some point in time the owner has purchased a scale replica of Thunderbird 3 (T3). This became more of an interest for me as I casually snapped a picture and put it on Twitter which has at the time of writing 458 likes and 295 retweets, more than any of my other tweets, odd how some things just take off! 🙂

The lady on reception had recently started work at the hotel, she had previously worked for the now-closed bar restaurant next door, I bet you know where I’m going with this. Around 40% of the hotel’s bar menu was unavailable, so burger and chips it was! later that evening when she was pouring me another G&T from an almost empty bottle, she said “Queens!”, I was taken by surprise by this! She explained if you call “Queens” when a bar person is pouring you a drink from an almost empty bottle they have to pour the remainder into your glass at no extra cost. Well, I had learned something new and beneficial before the training had even started.

Sunday morning, Simon and I walked over to the information desk in the terminal and we announced our arrival. Shortly after a young guy (you reach an age when everyone is a young person!) wander over and introduced himself as Lee, he signed us in and took us up to the training room where he had tea, coffee, biscuits etc waiting for us. A little later the other two people attending the training arrived, Alistair and Danny (who is also a blogger, check out his blog here), we all got on well and with Lee too which made for a fun day.

## ROCC AGCS Training

Lee started the day with a recap of the appropriate parts of CAP 413 and CAP 452 (ROCC). He commenced with the abbreviations (phonetic alphabet, numbers etc) and some definitions and when to use them; such as “Confirm” to request verification of Clearances, instructions, actions and information. We continued to run through the basics of the Readability Scale, Transmitting Technique and Time.

After a break, we went on to Aeronautical station callsigns and the type of service provided by each e.g. Tower means they provide an ATC service, Radio an AGCS… Now it was time for some theory, luckily for me I had covered this before when doing an amateur radio exam, this is not as bad as it may sound to some! VHF Propagation and interference, VHF under normal atmospheric conditions travels in a line of sight. To calculate it you use the following formula D = 1.23 √h where D is the distance in Nautical Miles, h is the height in feet above the ground. So for 3,000ft, the transmission will travel 67.3 nm.

Next, we covered QFE, QNH and QNE followed by Aircraft call sign abbreviations, when and how to abbreviate them, category of messages and their priorities, Unit of measurement, emergencies and how to handle them as an AGCS station, the role of D&D and their location.

After all of the above we moved on to  RT, when and what an AGCS can say and many examples were given, we covered relaying clearances, aircraft and vehicle messages and examples of handling an emergency call on frequency.

Lee provided lunch which put to shame the many courses I have been on with other much larger companies.

After lunch, it was exam time!
There is a 1hr, 24 question exam, unlike most other aviation exams this is written answers, not multiple guess, sorry choice. I found this very concerning being dyslexic, but it was not as bad as I had feared.

We then watched a video and had to pick up as many mistakes as we could, which was good practice for the last part of the ROCC, the practical exam.

Lee asked who wanted to go first and as I just wanted to get it over and done with I volunteered!

In a room on our own Lee explained how it would be run and checked I was clear on this. To be honest, at this point I was quite nervous, Lee read out a long message and I for the life of me could not write it down in time. Remembering my radio FRTOL training I decided to ask him to repeat the message and to speak slower, It worked and was permissible too. It was then just a case of remembering what we had done and what you hear most of the time when flying. All went well as Lee worked his way through about 5 different scenarios from an aircraft joining a circuit with other traffic, a fire truck crossing, relaying a clearance and checking the read back and lastly handling an emergency.

Yay, I passed and so did everyone else on the training course too, happy days!

## ATC Radar and Tower Visit

We now headed over to the control tower and spent some time in both the radar unit and the tower, which was very interest and we were made to feel very welcome, it transpires that controllers are normal people just like you and I, furthermore that they would prefer you talk to them and get the radio wrong than not to talk to them at all, this is something the Norwich ATC had said to Simon and I in the past too.

## Tower

As we all got on so well we are all planning to fly into Humberside in a few weeks time for a mini-reunion!

In summary, a worthwhile day, where we refreshed our radio calls, gained the ROCC qualification, had an awesome time looking around the ATC Radar and Tower and made some new friends as well!

I also believe that the ROCC may count towards my BMAA Gold wings award, bonus!

## Get Your Wings gets it’s BMAA Bronze and Silver Wings!

At the last (and my first) instructors seminar (Feb 2019) Rob Mott from the BMAA presented a scheme called “Wings“. It has various levels Bronze through to Diamond, each level becomes progressively harder to achieve, thus challenging the pilot to further their skills in a number areas Safety, Flying Skills, Education and Navigation (flying to a predetermined route). To be honest, at the time of presentation of the programme I didn’t think I would have the time or the inclination to embark upon the programme. One wet non-flyable day I took a look at the scheme and decided I had the required training within the 24 month prior period, so as I only needed to do the flying I decided to go for my Bronze, thus if anyone at my club or through my blog or twitter account asks about the scheme I would have first-hand experience of the scheme.

The Bronze award requires one safety item, for me this was the BMAA first Aid training which I did a while ago and would highly recommend this life skill to everyone, and a cross country nav with one land away and one waypoint on each leg, total distance needs to be over 100NM, each waypoint needs to be 20NM or more from the point of departure. Come the day it was very blustery but, within limits, the conditions were challenging. To make it more interesting I thought I would throw in a zone transit as in my normal flying and instructing I don’t get to do many of these so it was a good opportunity to practice. All went to plan and thanks to Norwich ATC I got my zone transit which meant I would keep well to time. Document completed before the flight and filled in during flight, times totaled and the fuel estimate compared, I was within limits (+-20%) so I emailed the paperwork into the BMAA.

It taught me that my fuel estimate was under, not by much just 2lt but I was expecting it to be over, (I blame the headwind and having to increase my speed to meet the plan!) I now allow a little more than before, a valuable lesson learned. I also enjoyed planning and flying to the plan. So I thought I would give my Silver Award ago too. I was surprised just how accurate I was on this as it was double the mileage (200NM) and two land aways. I was very pleased with a 99% accuracy on my fuel this time and the fact it was 1lt over, much better than being under as I was for my Bronze!

I may go for my Gold but, with the cost of the extra training and the time to fly it, that may have to wait.

I really enjoyed doing the Bronze and the Silver award and have just two things I will be feeding back to the BMAA, the first is I think it would be good to be made to list and plan fuel for at least one alternative airfield and the second point would be on the Silver and above maybe the provision on the form for two or more waypoints.

I would recommend following the scheme to all but, especially new pilots.

Some pictures from my Silver award flight below:

## A visit to Turweston

I had been meaning to take my daughter flying again for sometime, but with instructing and inspecting, time has been scarce!

Today was the day, but the wind was forecast to be gusting 15kts and 90 degrees across the runway, bang on the limit of the C42. However  as I needed to practice my crosswind landings I decided to go, after all I could always go-around if I didn’t like it and have another go before giving up and bugging out!

At Chatteris I PPR’d (Called for permission to land, Prior Permission Required), this was granted, then the guy said to keep a good lookout as they have two different fly-ins happening today,  I normally prefer a quiet destination.

The person who had our aircraft before me was late back, not his fault as he got caught by a couple of parachute drops, a hazard of flying from Chatteris.

The flight over was without event, PilotAware and SkyDemon were on top form and helped me keep the good lookout requested, which is also normal practice.

I joined downwind with other aircraft Joining crosswind, overhead, downwind and long final, this was going to be “fun”.

Silverstone Race track is adjacent to the downwind join for 09

As it happens the circuit was uneventful. I was high on final and with very little headwind I had to side slip to get down.

I must have had someone faster behind, as I heard them call going around as I was rounding out. This always use to worry me, but you just need to focus on flying and landing your aircraft. They are behind so its up to them to provide the appropriate separation and if they don’t or can’t its their fault not yours.

Santa Pod Drag strip on-route

On my three visits to Turweston I have always found the guys in the tower and on the radio to be excellent and very welcoming, as were the marshals and restaurant staff today. It has been a pleasure to fly there each time and I will be back for some more cake in the future!

On our return we had a Chipmunk push in front of us as we were waiting to go in the warm up area, but that’s fine  he was burning a lot more fuel then us!

On-in-all a very enjoyable day spent with my daughter #FatherDaughterTime!

## PilotAware and Skydemon the perfect VFR pairing?

I had an exciting and varied day Sunday, first I taught lesson 7 from the new syllabus (climbing and descending ) ending up with a bit of lesson 6 (straight and level) revision in poor conditions, where the horizon was barely visible. The student did very well in difficult conditions and its always nice when they say they really enjoyed it.

Today was for me about PilotAware and Skydemon, regular readers will know I think that currently, this is the best possible combination; during the lesson in the haze it was difficult to see other aircraft but, with PilotAware and Skydemon I could see the other aircraft, which we knew was out there, as it was from our club. The indicated position on Skydemon allowed us to quickly locate it and ensure we remained a safe separation. +1 for the combo.

Next up was my opportunity to occupy the co-pilot seat in a Twin Otter. The Twin Otter has Mode S transponder but, no ADS-B Out and no ADS-B In, thus they wanted to see PilotAware in operation. The Twin Otter in question was Skydive one the parachute aircraft, the test was very successful, in part because most aircraft at our home airfield have ADS-B Out or PilotAware. On the first flight we got a traffic service from Lakenheath as normal and shortly after we were showing a contact on Skydemon, the pilot said it will be interesting if ATC reports the same or not, just as he finished speaking or the radio came traffic information relating to the aircraft. This was interesting and helped show how pilot aware could help. On the next lift an aircraft was showing ahead and, on the left, Skydiveone would have normally turned left but, an alternative routing was taken to maintain separation.  Now the worth of PilotAware and skydemon was proving interesting! Then a little later in the flight we asked for clear drop and ATC came back with another aircraft in the zone, we had heard on the other frequency that an aircraft was leaving the circuit and with the traffic information we were able to positively ID the aircraft, thus there was no need to abort the drop. Fantastic news for both the safety and the cost of a go around!

There was few other contacts that helped too but, above was enough to convince them the effectiveness!

Then after a few lifts in the Twin Otter, I got to be the passenger in our C42A while my friend and co-owner did a few circuits.

## What is EC or should I say Electronic Conspicuity or even ADS-B In and Out?

What is EC or should I say Electronic Conspicuity or even ADS-B In and Out?

And why do we care anyway!

In today’s world of aviation, technology is becoming the norm and if that helps to increase safety then I for one am in favour of it and I hope you are too.

It was not long after I got my NPPL that I was using and EC in the form of the then beta product of an upcoming technology called Pilotaware. Fast forward a number of years and Pilotaware, SkyEcho 2, Stratux, Power FLARM and more are all affordable in cockpit EC devices. Add to these the traditional transponders which have various modes A, C and S as well as ADS-B Out and some systems having ADS-B In and then there is weather reception, TCAS and secondary surveillance radar (SSR) too!

So as a member of both the UK Microlight and GA community what is best for me and what do all the TLAs (Three Letter Acronym) mean?

ADS-B stands for Automatic dependent surveillance-broadcast, for your starter (yes I know that is a four-letter acronym).

But first to set the scene, I would not consider myself an expert, but I do have some knowledge having researched the subject to enable me to write my transponder decode application which is available for free download from this site.

The short version of this blog is that I remain happy with my current combination of devices, which consist of ADS-B Out, and Pilotaware for ADS-B In and Pilotaware device reception including OGN (Open Glider Network) when near a ground station that is up-linking and all displayed on SkyDemon.

## The Transponder

The transponder is a powerful device and comes with various modes depending on its age and capabilities. Older aircraft have Mode A & C with newer Aircraft/transponders having Mode S also

Mode A: In response to Mode A interrogation the transponder transmits an identity code for the aircraft in the octal range 0000-7777 the Squawk code.

Mode C: Transmits the aircraft’s pressure altitude automatically and augments mode A hence sometimes called mode A/C

Mode S: Mode S has over 17 million unique 24-bit aircraft addresses known as the ICAO number or mode S address allowing the unique identification of every aircraft, altitude reports in 25 feet increments and the call sign (or tail number) is transmitted along with other information also.

Unlike traditional Secondary Surveillance Radar (SSR) stations which elicit multiple replies containing the same information from all aircraft within their range, Mode S makes selective (Mode S is abbreviated from Mode Select) interrogations of each specific aircraft. ‘All call’ interrogations are also made to identify new aircraft to be interrogated. Mode S transponders are backward compatible with Mode A/C radars.

Civilian Mode S supports a number of different messages Each message has a particular purpose. The formats DF0, DF4, DF5, DF11, DF16, DF20, DF21 and DF24 are used in civil aviation at present.

 Number Message DF0 Short Air to Air ACAS DF4 Surveillance (roll call) Altitude DF5 Surveillance (roll call) IDENT Reply DF11 Mode S Only All-Call Reply (Acq. Squitter if II=0) DF16 Long Air to Air ACAS DF20 DF21 Comm. B Altitude, IDENT Reply DF24 Comm. D Extended Length Message (ELM)

#### Location Reporting

Now some people think as they have a mode S transponder it transmits their position and that their exact location is displayed to other aircraft, but this is not the case. Location information is transmitted in the ES (Extended Squitter) messages DF17 and DF18.

 DF17 1090 Extended Squitter DF18 1090 Extended Squitter, supplementary

For a mode S transponder to transmit ES DF17 (we will come to DF18 later) the transponder must be connected to a position source i.e. a GPS, and these come in different levels of certification from uncertified up. This configuration is known as ADS-B Out.

It is DF17 (and DF18) that devices like Pilotaware and SkyEcho use to show the exact location of other Aircraft. For non-ADS-B Out aircraft, they use the signal strength only to show proximity but, not location and they show altitude received too.

If only ADS-B Out gives the position how does Flight Radar 24 and 360Radar work their magic and show position of the other aircraft without ADS-B Out? Well, this is done by a system called MLAT which is out of scope for this blog as it requires multiple ground stations.

So DF18, this is essentially the same as DF17, however, it is the message used by a new breed of EC devices which are low power and low cost. They are regulated under CAP 1391. CAP 1391 (First published 2016) specifies Electronic Conspicuity devices that have the ability to signal their presence to other airspace users, thereby turning the “see-and-avoid” concept into “see-BE SEEN-and-avoid”.

It must be noted that CAP1391 states that they must not transmit if carried in an aircraft equipped with a Mode S transponder as this will result in mode S messages and DF 18 being broadcasted.

A system receiving DF18 implicitly knows it cannot interrogate that device, hence no aircraft can transmit DF18 if it has a mode S transponder as it will block or confuse SSRs (CAP1391 6.35). You cannot switch your mode S transponder off due to SERA.13001.

SERA.13001 requires the pilot of an aircraft equipped with a serviceable SSR (Mode S) transponder to operate the transponder at all times during flight.

Additionally, these new devices are intended for UK Annex II aircraft; non-complex EASA aircraft of <5700kg MTOM and for gliders and balloons. (CAP1391 Executive summary 7).

#### GPS

Mode S sends more information then just the location received from the location source, it sends information about how reliable that data is and if it were to fail, the size of containment that is required and more.  The data that most advertisers quote is the SIL value but, this is just one of the parameters.

Much is made of some of some of the new devices having a SIL1 GPS  in their ads, however, most have an SDA of 0 which means they are not trusted!

SIL (Source Integrity Level) field is used to define the probability of the reported horizontal position exceeding the radius of containment defined by the NIC value. 0 is unknown integrity or untrusted!

SDA (System Design Assurance) field defines the failure condition that the ADS-B system is designed to support.

NIC (Navigational Integrity Category) is reported in conjunction with the SIL, NIC of 0 is unknown integrity or untrusted!

#### Can I be seen on TCAS?

Yes, if you have a mode C transponder because most TCAS system utilises an aerial array to determine the position of other aircraft using mode C transmissions.

TCAS II has to be linked to a mode S transponder if fitted so its presentence is encoded in the mode S messages. But TCAS systems below version 7.1 don’t use the ADS-B out messages from other aircraft.

TCAS II Hybrid Surveillance does use Mode S messages but currently has limited adoption.

Hybrid surveillance is a method that decreases the number of Mode S surveillance interrogations made by an aircraft’s TCAS II unit. This feature, new to TCAS II version 7.1, may be included as optional functionality in TCAS II units. TCAS II units equipped with hybrid surveillance use passive surveillance instead of active surveillance to track intruders that meet validation criteria and are not projected to be near-term collision threats. With active surveillance, TCAS II transmits interrogations to the intruder’s transponder and the transponder replies provide range, bearing, and altitude for the intruder. With passive surveillance, position data provided by an on-board navigation source is broadcast from the intruder’s Mode S (ADS-B Out) transponder. The position data is typically based on GNSS and received on own aircraft by the use of Mode S extended squitter, i.e. 1090 MHz ADS-B, also known as 1090ES. Standards for Hybrid Surveillance have been published in RTCA DO-300. The intent of hybrid surveillance is to reduce the TCAS II interrogation rate through the judicious use of validated ADS-B data provided via the Mode S extended squitter without any degradation of the safety and effectiveness of TCAS II.

Should you buy a CAP 1391 device such as a SkyEcho 2? Well that’s up to you but, if you have a Mode S transponder you will need to disable the only feature it has over Pilotaware, which costs less and has mode compatibility with other systems.

If you don’t have a Mode S transponder then Yes, it is an option and will increase your visibility, but not to most TCAS equipped aircraft including military jets and helicopters as they are looking for mode C transmissions.

## Conclusion

Finally, for me a proper transponder with ADS-B Out (certified GPS or uncertified) is also essential in the increasingly busy skies for your visibility and Pilotaware as your ADS-B In device gives you the best low-cost solution currently.

Today I was told a story of an instructor doing a PFL and they got a Mode C alert on Pilotaware but, at first could not see the other aircraft. Then at a low level saw a chinook below them. The instructor’s aircraft also had a transponder with Mode A/C and S also with ADS-B out, so the TCAS in the Chinook would have been going off too. “see-BE SEEN-and-avoid” in action.

## A trip to the Isle of Wight, Sandown

The Isle of Wight was a trip I have long wanted to do and was also a standby option a few weeks ago when I flew to Le Touquet with my friend Jason, Jason and I were to make this trip too. Compared with Le Touquet this would be a breeze, no flight plan to file, no GAR to file either and far less airspace to transit.

We set off about 10:30 and flew down south and over my old hometown of Reading staying below 3,500ft to avoid the London TMA, in fact, we stayed around 2,000ft as we were close to a part of the TMA which started at 2,500ft and didn’t want to risk an accidental infringement.

Next was Odiham MATZ, we called and asked to a MATZ penetration and this was granted and they also suggested we switched to Farnborough West Lars on 125.250 which we did, calling them  they confirmed our MATZ penetration and advised us of intense gliding at Lasham, so we routed to the west of Lasham and gave them a wide birth! this was also a good test for Pilotaware and Skydemon which showed us lots of other aircraft including gliders.

Clear of the MATZ we switched to Solent Radar, I must say the guy on the other end of the radio was great. We crossed the Solent at Portsmouth with our declared routing as “direct to Sandown”, but we decided to do a lap of the Isle of Wight and take in Cowes and the Needles. As we turned the guy on the radio came back to us and asked if we are intending to fly around the Isle of Wight, we said if that was ok, yes, it was so we carried on. We were not within controlled airspace at the time so it was ok to change the routing.

As we approached Cowes we asked for clearance to enter controlled airspace and this was granted. As soon as we passed into the CTA we were told we were now under Radar service and cleared to follow the coast, I guess they get a lot of us pilots doing this!

### John Cleese or the Fonz!

Our landing was uneventful and we parked up and went into the cafe, where we were made to feel welcome; we ordered the traditional bacon roll each and two coffees. This is where things took a comedic turn as the lady started hitting the coffee machine as if she was the Fonz. Fonzie was clearly not in the house as the coffee machine silently took the hits without giving up any coffee, eventually, a guy turned up and took over giving the coffee machine a beating (this was now more John Cleese then Fonzie)! At last, it gave up and two coffees were dispensed.

The journey back was a reverse of the flight down and we arrived safely back at Chatteris a little before sundown.

### Some of the views of the on our trip to the Isle of Wight

I can recommend a trip to the Isle of Wight to anyone, there is not too much radio work and great views!

## First channel crossing and visit to Le Touquet

I had been wanting to fly an aircraft from the UK to another country for some time. Almost a year ago when chatting to Jason an AFI at Connington I met while doing my AFI (or FI(R) ) training I mentioned this to him. Jason had already done the trip once and thus he knew what to expect.

We planned to fly on the bank holiday Monday 27th August 2018, but the weather was not looking good,  the day before it had improved to be around the limits of the aircraft.  We decided to proceed and plan the trip in the hope it would in improve further and we would get to go. We met up with Paul who was going to follow us over with Tom in their C42, as they had never done the crossing either.

The first thing to do when thinking about doing a crossing is to order a life jacket. I went for the Crew Saver N180 form the Marine Superstore. I found it comfortable and not bulky; also the guy there was helpful when I called them and it arrived promptly. The main things to check when buying a life jacket is that it MUST be manually operated and the buoyancy is at least 150N.

https://www.marinesuperstore.com/lifejackets-buoyancy-aids/lifejackets/crewsaver-crewfit-180n-pro-lifejacket

## Planning the trip

We met up in a Costa coffee (other coffee shops are available!) to plan our route from Chatteris to Le Touquet.  We decided to route across and down to South End and then on to Dover and across the channel at its narrowest to minimise the risk. It can get congested in the skies over Dover, however,  as we were leaving early it should be ok.

Next was the form filling this was so simple using SkyDemon. Simply buy some flight plan credits, I think this was £5 for five.

Do your route in SkyDemon and hit file plan, you need to add a few extra details and the flight plan is done for one leg, Reverse the route and resubmit for the return, flight plans filed, yes that’s it!

Next, the GAR wait, SkyDemon does that too, just go into the warnings and click on the spanner next to the warning re needing a GAR, complete the form and hit submit, I LOVE SkyDemon. Last bit of paperwork was to send a letter to Le Touquet customs, there are plenty of examples online. But basically, give them the reg, date, ETA, and the passport numbers of all people on board.

On the day the wind had dropped a knot or two and now was just under the limit so we decided to try the flight. If the cloud looked bad we would divert and stay in the UK. Nothing good ever comes from pressing a bad situation.

## Flying out

We set off at 07:50 with Jason on the radio and me at the controls, we climbed to 2,000 feet, this allowed us to call London Information and activate our flight plan. The call was as simple as “Call sign, London information, request basic service a flight plan activation” and then the normal pass your message and its response.

A little while later we were asked for our ETA for our arrival at South End airspace, this threw us a little but a quick look at SkyDemon’s Plog gave us the information we needed.

As we were getting nearer to South End’s airspace and thinking about changing frequency, we got a call from London Information advising us to change frequency and to call South End and that they would speak to us again once we have left South End’s airspace. We called and asked for a zone transit and this was granted and squawk allocated, back came the call to squawk the same code as we were already squawking! Apparently, it was still transmitting the last squawk, we recycled the transponder and all was ok. We flew straight through with a radar service, out the other side, they downgraded it to a basic service and they advised us to call London Information. This was all very simple!

Dover

As we approached Dover the cloud was driving us down from 3,000ft to around 2,000ft and the crossing was beginning to looking in doubt. However, as we cleared the coast it was brighter and we crossed at 2,500ft with good visibility. The French coast soon came into sight so we continued, at around halfway we called London information and requested a frequency change to Le Touquet which was approved. The call was made as we approached the boundary as marked on SkyDemon.

We routed down the coast and call Le Touquet, back came a pass your message with a heavy French accent! The normal response was given back and then we got “report passing November”, passing November, it was only August! a quick look at the plates we had printed out to see if we could find visual reporting point (VRP) November didn’t yield anything, then we saw that SkyDemon have these on their map, which again made it very easy.

At November we called and was told to report November Bravo. NB was close to the downwind leg of runway 31 the runway in use. Using the SkyDemon approach helper, we entered the runway, direction and downwind join, this shows you the circuit, which again at an airfield you don’t know is very helpful.  I would and I did a quick sense check to ensure it seemed correct.

As we got to November it started to rain, 10% chance the weather report said, I’m not that luck normally! It continued to rain on the downwind leg, but bizarrely it stopped as we turned on to base leg.

The wind was now gusting 20kts, but only about 30 degrees off the runway. I decided to use a single stage of flap and we landed without issue, funny how nobody ever sees your good landings and doubly so in difficult conditions!

After clearing passport control we went to the reception and paid our landing fees and purchased 20lts of Avgas.

We walked into town and found a nice bar to eat, the place, in general, seems to be friendly and there are a few places even flying the Union Jack flag! A nice little town and I will go back at some point, next time as the radio operator!

### Return

The return was nearly without event the takeoff was with a crosswind and gusty too, it was very bumpy on the climb out.

The crossing was fine and the white cliffs of Dover welcomed us back home. While crossing South End’s zone we were controlled both by altitude and direction to allow for an incoming jet, this was the first time I have been given a heading and altitude to fly while crossing. A little while later came the call to resume self-navigation, unsure if this meant if we could now climb or not we asked and was told to remain not above 1,500ft and they would call us when we could climb, another jet passed by and then we were called and allowed to climb not above 4,500ft.

We were soon near Chatteris so we called for a frequency change and to close the flight plan, frequency change was approved but we were told the plan cannot be closed while we were in the air.

My landing at Chatteris was odd a little before the threshold I put a little power on as I was a bit low,  but just as I did the aircraft went up as if it had ballooned, I landed fine and said to Jason it was odd as I had not pulled back on the controls. Later we watched another C42 returning from somewhere and it did the exact same thing, then Paul said he had seen it happen to me so was ready for it but it caught him out too!

In summary a very enjoyable experience, but a very long day and very tiring too!