Safety Matters and Safety Briefs are based on the AAIB Bulletin and UK Airprox Board reports, with additional material from the US National Transportation Safety Board
Aircraft Type: DH Tiger Moth Date & Time: 26 August 2017 at 0828 Commander’s Flying
Experience: PPL (former RAF pilot and ATPL), 22,240 hours, 512 on type Last 90 days: 50 hours Last 28 days: 24 hours The Compton Abbas-based aircraft was making its first flight of the day, a thirty-minute introductory trip for the passenger, who had no flying experience. Takeoff was from Rwy 08 and witnesses saw the Tiger Moth become airborne at about the 300m marker, at which point its engine was heard to misfire, but the aircraft continued to climb. The airfield does not record transmissions on its A/G frequency, but the operator heard the pilot transmit in an apparently calm voice that he had a “rough running engine” and was making a 180° turn to land back on Rwy 26. During the turn, or when rolling out of it, the aircraft appeared to slow and become unstable before a wing dropped sharply and it descended rapidly in a turn which a witness thought was to the right. Another witness heard and then saw the aircraft climbing slowly at low airspeed, with the engine “sounding awful and misfiring” before it descended, “corkscrewing down”.
A flying instructor who was next to the radio operator when he heard the transmission indicating engine trouble saw the aircraft heading north some 1-2nm from the airfield at a height of about 500ft, then make a gentle, descending left turn onto right base leg for Rwy 26. When it was at about twice the height of trees at the eastern end of the airfield, and some 200m beyond them, its nose pitched sharply down and it rolled to the left in a steep descending turn. Shortly afterwards a column of smoke was seen.
The Tiger Moth had crashed in a cornfield in a steep nosedown attitude. Post-crash fire consumed most of its fabric covering and cockpit structure, including the seats, both safety harnesses, instruments, instrument panels and electrical wiring. A pathologist subsequently concluded that the impact injuries sustained by both occupants prior to the fire had not been survivable. AAIB examination of the wreckage showed that the flying controls had been connected prior to the accident, but it was not possible to determine their positions at impact. The engine had suffered impact and fire damage, but there was no evidence of mechanical failure prior to the accident.
As part of the investigation two trial flights with Tiger Moths were made to establish takeoff, climb and descent performance at different airspeeds, at full power and with the throttle closed, and with varying angles of bank in the turns. From this data height gained or lost against time and/ or distance could be calculated. It was not possible to determine the effects, if any, of the pilot’s report of a “rough running engine” on the performance of the accident aircraft, but rates of climb recorded in the trials were greater than those estimated for the accident flight. The final two test points simulated turnback manoeuvres assuming a worst case in which the power available from the engine was equivalent to idle. The test pilot climbed to 800ft and closed the throttle, lowered the nose to achieve a target speed of 66mph, and rolled the aircraft to a 45° angle of bank to start a simulated turn back to the airfield.
He found that both the Tiger Moths flown had similarly benign stall characteristics at between 45-49mph IAS, adopting a high angle of attack and rate of descent with the stick held fully back, and showed no tendency to drop a wing. The measured rates of descent were consistent with other Tiger Moths he had flown, but with a noticeable increase when flown at or just above stall speed. The aircraft could be turned relatively easily with an increasing rate of turn, approximately 45° bank being the optimum for a turn back towards an airfield. Height loss in turns through 180° was approximately 400ft. The test pilot considered that starting such a manoeuvre much below 800ft, following a total engine failure, would have given the pilot little opportunity to manoeuvre onto the reciprocal runway.
A third trial was flown to assess rejected takeoff performance, climb performance at partial power/ low rpm, and aircraft attitude in a glide at minimum rate of descent. The test pilot reported: “Rejecting the takeoff in the Tiger Moth introduced a number of challenges. The aircraft was not fitted with wheel brakes, had a very crude steerable rear tailskid and was a ‘taildragger’ configuration. Rejecting the takeoff when the wheels were still in contact with the runway required the throttle to be closed and the level attitude held initially before allowing the tail to drop gently as the airspeed reduced. During this period there was no braking, and steering was entirely aerodynamic and reliant on airflow over the rudder (which was reduced with loss of propeller wash). This technique worked satisfactorily and, from being light on the wheels at 40KIAS, the aircraft was brought to a halt in approximately 230m after closing the throttle and following an initial takeoff run of 200m.
'The situation became slightly more problematic once the aircraft had become fully airborne. Typically, the aircraft would be landed in a "threepoint" attitude, main wheel and tailskid contacting the runway at the same time in a stalled or semi-stalled condition with minimum ground and airspeeds. Rejecting the takeoff once airborne at 40+kt IAS meant that the initial application of aft control stick would have potentially caused the aircraft to climb before the "three-point" landing attitude was reached, which would have resulted in a heavy landing. The alternative was to retain the level attitude and close the throttle, allowing the aircraft to descend back onto the runway initially on the mainwheels and then complete the reject. Any delay in making the decision to reject once airborne would noticeably extend the distance required to complete the manoeuvre [and] given the risks of performing such a manoeuvre once airborne it is unlikely that it was practised regularly, if at all. This type of reject was attempted during the sortie when the aircraft had reached about 20ft agl and a flying speed of 50kt IAS. As the throttle was closed and the nose dropped to a level attitude for landing… insufficient runway remained to complete the landing without risk, and power was re-applied to climb away.'
The AAIB report concludes: ‘On takeoff the engine was
heard to misfire, but the aircraft continued to climb. It would have been possible to abandon the takeoff, closing the throttle and attempting to stop on the runway. If the decision was made promptly there would have been sufficient space remaining for the aircraft to stop or turn away from the obstacles at the eastern boundary. Once the aircraft was airborne, the pilot would have faced the same situation observed by the test pilot during sortie three of the flight trial. During the accident takeoff, engine speed reduced to 1,710rpm. This may have been a result of the pilot reducing throttle in order to reject the takeoff. However, the takeoff continued and he may have concluded that there was insufficient runway or other space on the airfield in which to stop.
‘Flight trials indicated that power settings 1,700 and 1,800 rpm would not have been sufficient to achieve the height gained in the time available. Although the pilot had reported “rough running”, the engine must have been producing sufficient power to achieve between 1,900-2,000rpm at the best rate of climb speed in order to gain between 300-500ft above airfield level in the time available.
'Subsequently, both the turn and descent back towards the airfield were gentle manoeuvres, and there were no additional transmissions from the pilot reporting any deterioration in his situation. Witness evidence varied regarding the flight path, distance from the airfield and the direction of the turn back towards it… Up to this point, the gentle rate of descent suggests that the engine was still producing power but was not, and probably could not be, heard from outside the flying club if at reduced power. The pilot either made a normal approach, throttling back and descending on base leg or, if power available had reduced and he was unable to maintain height, was forced to let the aircraft descend. With a rough running engine… a total or increased loss of power is always a possibility, and maintaining height until within gliding distance of the intended landing point is desirable in this situation if it can be achieved whilst maintaining sufficient speed.
‘The aircraft’s nose was seen to pitch positively down some 30°, which when combined with the turn, would have caused the aircraft to lose height rapidly. Flight trials established that the rate of descent could have been at least 1,200ft/min if the aircraft was just above the stall at low speed and low power.
‘Three possible reasons for the final manoeuvre were considered: (1) Low airspeed resulting from trying to maintain height which caused a wing to drop at the stall, as described by one witness; (2) Low airspeed resulting in the need for a large nose-down attitude change in order to regain airspeed but resulting in a significant rate of descent; or (3) A late decision to change the landing area to the crop field, requiring a tight left turn. This would have required an increase in airspeed, obtained by the significant nose-down attitude change before the turn. Either (1) or (2) may have caused the aircraft to descend below the minimum glide angle required to land at the airfield. In these cases, the pilot may have recognised that he was not able to return to the airfield and turned towards the large open area of crop field. Alternatively, the angle of the descent placed the aircraft at a height and position from which the pilot decided to change his landing area to the crop field. He may also have been attempting to recover from or avoid a loss of control at low airspeed.
‘Damage sustained during the accident prevented an assessment of the magnetos, ignition system and carburettor. A problem with any one of these could have caused the reported symptoms. The possibility of carburettor icing could not be discounted. Finally, there was either insufficient height to recover, or a restriction of the controls that prevented recovery before impact. The accident aircraft was fitted with dual controls and although the passenger had received a full cockpit and safety brief, it is possible the flying controls were involuntarily restricted as the passenger reacted to what would have been a very stressful situation.’
Aircraft Type: Piper Cherokee Archer II Date & Time: 13 June 2018 at 1148 Commander’s Flying Experience: PPL, 387 hours, 347 on type Last 90 days: 7 hours Last 28 days: 1 hour The pilot had planned to fly to Skegness for a day trip with a friend. He telephoned the airfield for PPR and was told that A/G radio was not operating that day, but having been there before he decided to make the flight, intending to land on the 799m Rwy 03/21, the longer of the airfield’s two grass strips. On arrival the pilot saw no windsock and, despite making two level circuits at 1,000ft, neither occupant could see the runway’s ground markings, so he decided to use the shorter, 650m 11/29 strip, which he could see.
He flew a left-hand circuit for Rwy 11 at 1,000ft and turned final at 700ft, but towards the end of the approach realised that he was going to land far down the runway so he went around. As he did so the aircraft clipped a hedge beyond the departure end of the runway, and some foliage became attached to the airframe. The pilot struggled to maintain control and prevent the aircraft from stalling, turned right “with difficulty” to avoid a caravan park ahead and flew a nonstandard right hand circuit over clear ground. He stated that the aircraft would only remain airborne with full power selected, and while rudder and elevator appeared to respond normally, the ailerons were operating “strangely”. Due to the high power setting, the Archer touched down at around 90kt. Braking had little effect, and as it neared the end of the runway the pilot shut down the engine and told the passenger to brace, then slewed the aircraft to the right so as to avoid some trees situated within the same hedge that had been struck on the first landing attempt. The Archer came to rest in the hedge, which blocked its doors and prevented pilot and
passenger from escaping until rescuers arrived about fifteen minutes later.
The pilot reflected that his choice of runway was a factor in the accident, and that the winds appeared to be more westerly than he had originally thought. Prior to the flight, he had checked runway information on the Skegness Airfield website, which describes them as: ‘Runways 03/21 799m x 23m, 11/29 650m x 23m. All circuits inland: 03/11 LH, 21/29 RH’.
Notes the AAIB: ‘The description of the runways and the circuit directions may have contributed to some confusion by the pilot over the runway designations. Since the accident Skegness Airfield stated that it intends to alter its website to emphasise the runway designations.’
At its December meeting the UK Airprox Board reviewed 29 incidents, nineteen of which involved drones/uass. Among the aircraft-to-aircraft incidents, four had a definite risk of collision (three Category A ‘with providence playing a major part’, and one Category B where ‘safety was much reduced as a result of serendipity, misjudgement, inaction, or late sighting’).
Up to the December meeting, at 181 the overall number of aircraft-to-aircraft incidents in 2018 was just above the expected five-year annual average. In contrast, there were 132 reported drone/suas incidents, well in excess of the 113 recorded in 2017.
‘Predominant themes were poor lookout and non-/latesightings which… accounted for six incidents,’ says the Board. ‘Given that most incidents occurred in see-and-avoid Class G airspace this isn’t perhaps surprising. However, in at least two incidents the Board felt the subject pilots should have been doing more to enhance their chances of seeing the other aircraft by paying more attention to lookout.
That said, the vagaries of the human eye in the aviation environment were acknowledged.
‘Inaction featured in three incidents where pilots could have done more when becoming aware of the other aircraft. One was particularly disappointing when a pilot didn’t give way because he thought he would carry on rather than cause his student problems by deviating from his student’s navex track.’ The Board emphasised that ‘avoiding’ doesn’t necessarily have to be in just the horizontal plane – climbing or descending would have been an option in this case if it was important not to overload the student by turning off track.
The remaining incidents showed no clear trends except ‘where pilots could have helped themselves by seeking a more appropriate air traffic service other than a Basic Service (or no ATS)’, says the Board. We’ve talked many times before about the value of a surveillancebased Traffic Service, but the message still appears to fail to register with some who either anticipate they will not get such a service, so don’t even ask, or who think that they will receive Traffic Information when under a Basic Service (which they might, but which is not the intention or expectation with that level of ATS).’
Analysis of 116 incidents last year assessed prior to the December meeting showed that ATC was not available for 26% of them. ‘There’s not much we can do about that in the short-term,’ says the Board, [but] the striking statistic is that ATC were not used to provide Traffic Information when it might have been available in 31% of the incidents. Although this includes times when a surveillance-based service was not available, a good proportion of these latter incidents include ATC not even being called up, or pilots asking for only a Basic Service when a Traffic Service could have been used.’
The Board’s ‘Airprox of the Month’ involved a Piper PA-28 and a Christen Eagle, both inbound to White Waltham (illustration above). Both were joining overhead at the standard modified overhead join height of 1,300ft parallel with the runway. ‘The faster Christen Eagle came in behind the PA-28, but its pilot hadn’t assimilated the Piper pilot’s calls as he joined ahead,’ says the Board’s report, ‘[and] the Piper’s pilot had heard the Eagle pilot’s joining call but, although aware there might be a conflict that he would need to be cautious about, he was taken by surprise when the Christen Eagle overflew from behind, descending and missing him by only a few feet. The Eagle pilot didn’t see the Piper but, fortunately, the PA28 pilot’s student had allowed their height to decrease to 1,230ft by mistake, otherwise the encounter could have ended in disaster.
‘Although a missed radio call is something that happens now and again, we all need to remember during our pre-joining checks that missed calls or even other radio-failure joiners are always a possibility that needs to be factored in to your decision-making process. As you approach the “honey pot” of an airfield where everyone is likely to be following the same track or routeing to the same point at the same height during the join, it’s vital that lookout efforts are redoubled for just such a reason.
‘Don’t let yourself become taskfocused on the procedure itself, especially if you’re in an aircraft with restricted views ahead such as the Christen Eagle.
‘Alongside making all the right calls in the right place and following the prescribed route, weaving the nose, dropping a wing to mitigate blind-spots, and meticulous attention to clearing your path ahead, above and below, are all essential elements of any airfield join.’