my iNfriNgEmENT

Richard Warriner reports on a vertical infringement of Stansted’s CTA – whilst approaching Barkway VOR

Conducting an instructional flight from Tatenhill to Headcorn via BKY – BPK – LAM, I managed to infringe the bottom of the Stansted CTA near BKY. The flight was conducted in a PiperSport, which has a glass cockpit and a three-axis autopilot. Approaching BKY, cruise altitude was reduced from 2,400ft to 2,100ft to allow some extra margin below the 2,500ft CTA floor because of turbulence. The aircraft was being flown on autopilot in

HDG/NAV and ALT modes and at the time we were monitoring the autopilot to ensure that the turn at BKY was executed correctly and that we didn’t infringe the

Stansted Class D airspace. At some stage approaching BKY, turbulence caused the disconnection of the ALT mode. There was no notification other than the extinguishing of the rather dim lights on the ALT mode switch. I noticed the aircraft altitude was increasing and at 2,400ft instructed the pilot to ‘descend, now!’. I didn’t see an altitude greater than 2,450ft on the altimeter, however Farnborough LARS asked us to confirm our present altitude as our Mode C was showing us inside controlled airspace. A couple of days later, the aircraft owner received an email from NATS asking the PIC for that flight to fill in an online questionnaire. A little later the same request was received from the CAA. In both cases I returned the questionnaires as soon as possible and awaited the responses with some trepidation. At the LAA Rally I took the opportunity to have a chat with the Infringements Team on the CAA Stand. This proved useful, as I could add some information and context to the basic report. One point was that the Infringement severity level appeared to be raised, possibly due to a ‘Loss of Separation’. As advised by the Infringement Team at the Rally, an email duly arrived on the following Tuesday saying: “The CAA now looks upon this matter as closed. However, a copy of this letter will be placed on your licensing record and may be referred to if any further reports are received in the future.”

Above Left is a SkyDemon trace of the route from Tatenhill to Headcorn, and right the section of the route approaching BKY where the infringement occurred

Thoughts and learning points

It seems that this incident was classed as a ‘Loss of Separation’ as there was IFR traffic less than 3,000ft above us. As we were receiving a Basic Service from Farnborough LARS (North) with a discrete squawk, I’d assumed we were ‘known traffic’ to Essex Radar, but that wasn’t the case. Whilst we were known traffic to Farnborough Radar, and other ATC units would have been aware that we were receiving a service from them (due to the squawk), units such as Essex Radar would not have been aware of our flight intentions or route. As such we would have been ‘unknown’ to them. Air Traffic Controllers are required to ensure safe separation is maintained between traffic under their control inside controlled airspace and any infringing aircraft, that by definition may be unknown to them at that

point. In the area that was infringed, the Stansted Controller was required to provide a minimum of 3,000ft vertically from the unknowns, and hence separation was lost. Had we been receiving a service from Essex Radar (unlikely given their traffic load) we would have been ‘known traffic’ to them, but there is no prescribed separation between IFR and VFR flights in Class D airspace. Traffic information is however, required to be provided to both flights. Given that about the only thing we get from a Basic Service is the QNH, the alternative services are probably a more useful option. Frequency Monitoring Codes (FMC, aka Listening Squawks) are really useful in this respect, as they enable the aircraft to be contacted rapidly by the actual controlling unit and allow for timely and effective resolution to any potential infringement. It’s not ATC’s job to keep non-cleared traffic out of controlled airspace, but it might have been better to have been using the Stansted Listening Squawk, or to have been in receipt of a Traffic Service from Farnborough. This NATS Blog* by an Air Traffic Controller indicates that the listening squawk and appropriate frequency dialed in could be a good option for anyone close to controlled airspace. A quick call from ATC could reduce the number or severity of infringements and save everyone from a load of stress and paperwork. While not wishing to add to controllers’ workloads, it would then be interesting to know how many ‘Saves’ (Infringements prevented by a quick call to an aircraft using the listening squawk), have been achieved. This information might encourage more pilots to use the system, particularly those who don’t like talking to ATC, as ATC does endeavour to transmit a warning to traffic using their listening squawk that appears to be about to infringe. However, this is very much subject to their workload and a warning cannot always be guaranteed. Where listening squawks definitely do help is in the rapid recovery from inadvertent infringements. ATC has a system called the Controlled Airspace Infringement Tool (CAIT), which shows up an aircraft that has entered controlled airspace without a clearance. If this could be modified to indicate aircraft, which on present trajectory would infringe, a quick call could save a lot of problems. The problem with GA operations outside controlled airspace is that their flight paths are not ‘reasonably predictable’ and ATC generally have no intention data (flight plan, route etc). ATC can extrapolate the track of the aircraft using radar data (if the flight continues on the same track, speed and vertical rate, where will it be in X minutes?). Most modern ATC radars can do this and provide the ATCO with a ‘predict vector’ on the screen. Automating this system is potentially problematic though because unfortunately, due to the random nature of some GA flight paths, especially near controlled airspace boundaries, it would generate far more false warnings than would be useful. Some light aircraft have sophisticated avionics, as with the autopilot in this case. Perhaps we need to be more familiar with the operation and failure modes of these systems. This can be a bit of a ‘gotcha’ for instructors flying in a pilot’s own aircraft where they are not familiar with the particular setup. A contributing factor is that on the autopilot in the PiperSport, it only starts the turn onto a new heading having reached the waypoint, hence the need to monitor the turn and that the rate of turn kept us within the four nautical mile gap between BPK and the Stansted Class D airspace. Horizontal positioning has had millions of years of evolution, vertical positioning is a more recent skill! Thanks to the CAA Infringement Team and NATS for their help with this article. ■

Above A fabulous panel (not from subject aircraft), but should you be relying on it to keep you out of trouble if you do not realise the limitations of the various component parts?

*NATS BLOG: h t t p s : / / n a t s . a e r o / b l o g / 2 0 1 9 / 0 3 / a i r s p a c e - i n f r i n g e m e n t - s e r i e s - c o n t r o l l e r s - s t o r y /

■ About the author: Richard Warriner is the well-known owner of a Rans S6 G-BUWK, in which he has flown 5,700 hours to date, averaging 340 hours per year over the last six years. He is also a Class Rating Instructor (CRI), keen to help members improve their flying skills and confidence as well as oversee their biennial revue flights.