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
Unexplained power losses, windshear effect, and a fatal Mustang go-around
Mustang’s fatal go-around
Aircraft Type: North American P-51D Mustang Date & Time: 2 October 2016 at 1434 Commander’s Flying Experience: PPL, 1,965 hours, 760 on type Last 90 days: 21 hours Last 28 days: 6 hours The aircraft was returning to its owner/pilot’s private strip at Topcroft Farm, Norfolk after a 45-minute flight with a passenger in its rear seat. A witness on the airfield saw it with landing gear extended and flaps fully down on a stable final approach to the 825m long grass Rwy 28 but with little, if any, attempt to compensate for the crosswind by either sideslipping into wind or flying with the into-wind (right) wing slightly down. At the time the airfield’s weather station recorded wind direction as NNW, average speed 13kt, maximum 22kt from the NW, which equated to a maximum crosswind component at the strip of approximately 18kt — not considered excessive for the pilot’s experience on type.
The Mustang touched down in a three-point attitude, bounced back into the air and drifted left towards the runway’s edge. A small amount of left roll was corrected with aileron. It then touched down again, bounced a second time, drifted increasingly to the left of the runway, narrowly missing a tree standing in an adjacent field and remaining at low level until it collided with another tree on the boundary between the two fields and struck the ground.
One of its propeller blades had been the first point of impact, slicing upwards through the trunk and into a bough of the tree which then detached, leaving the blade’s tip deeply embedded in the trunk. The aircraft continued onward, leaving a 57m long trail of debris leading from the tree to the main wreckage, resting on its right mainwheel and tail wheel, in the field beyond. The engine was partially detached from the fuselage and the propeller and its drive gear had separated from it.
Damage to the tree showed that the engine had been producing high power on impact. The left wing root then hit the tree causing wing and flap to detach. The bending and distortion to structure around the wing root and tailplane attachment suggested that, at impact, the Mustang was left wing low with a roll angle of approximately 60°. As it continued forward, the remaining right wing would have produced rapid left rolling moment, at which point the nose dipped and the aircraft hit the ground on the upper left side of its fuselage. Its canopy detached during impact and landed nearby, along with the remains of the left self-sealing fuel tank. The Mustang then bounced on to its nose at a near vertical attitude whilst rotating. Inertia and rotation then caused it to carry on tail-first, finally hitting the ground pointing in the opposite direction to travel. The impact with the tree at the left wing root had released the self-sealing fuel tank and its fuel ignited. The majority of fire damage occurred in the remains of the tree and the ruptured tank, burning fuel appearing only to have flashed over the front position of the cockpit just before or as the canopy detached. Pilot and passenger remained securely strapped in their seats during the accident. The pilot survived the impact but suffered a number of serious injuries, particularly to his neck, and burns to his face and neck. The passenger was fatally injured.
The pilot had no recollection of the accident but was able to describe his usual technique for landing the aircraft and going around, which would be to land with full flap selected. He would not consider it necessary to reduce the degree of flap selected when landing in a strong crosswind. He would also open the canopy during approach to facilitate evacuation in case of an emergency. He reported that he had carried out a number of go-arounds in the past, including some at Topcroft Farm, although it had been two or three years since he had last done so. He described his technique as applying power to about 40in Hg, keeping the aircraft close to the ground, and raising flap by 10° to reduce drag. As speed increased he would then continue to raise the flaps one stage at a time until at a suitable speed he would climb away and raise the gear. The original USAAF P-51D Flight Handbook provides the following instruction for a go-around: Open throttle smoothly. Do not exceed 61in Hg, 3,000 rpm Maintain wings level and nose straight Landing gear handle up Raise flaps slowly when at least 200ft above ground. The AAIB comments: ‘The P-51D has a fast responding engine mounted on the aircraft centreline giving large torque in relation to aircraft weight. This makes it liable to torque roll, an effect where, should the throttle be opened quickly, it causes the aircraft to roll in the opposite direction to propeller rotation. [In this case it] would result in a roll to the left. An aircraft is particularly susceptible to this during a go-around from an approach when power is increased from, or close to, flight idle, and the aircraft is less controllable due to its low speed. Corkscrew effect is the name given to the effect of the propeller slipstream which spirals around the aircraft’s fuselage. At low airspeeds and high propeller rpm this produces compact spirals which can exert a strong sideways force on the aircraft’s vertical tail surface. [On the P-51D], due to the direction of its propeller’s rotation, this causes a yawing moment around the vertical axis to the left. The rotating propeller has properties similar to that of a gyro. When a force is applied to a gyro, the resultant force acts at 90º ahead of, and in the direction of, rotation. Any action on the aircraft causing the propeller to change its plane of rotation also results in a force creating a pitching moment, a yawing moment, or a combination of both depending on the point at which the force was applied. This action is more prominent in tail wheel aircraft and most often occurs when the tail is being raised during takeoff, or a go-around. This change in pitch attitude has the same effect as applying a force to the top of the propeller’s plane of rotation, creating [in this case] a yawing effect to the left.
‘During the approach the aircraft was subject to a crosswind from the right for which the pilot did not adequately compensate. The situation was compounded by the direction of the circuit, for noise abatement, which resulted in the aircraft being on the upwind side and therefore ‘blown out’ of the circuit as it joined the approach. This culminated in the aircraft landing to the left of the centreline. It touched down at the appropriate speed and attitude but bounced, which further subjected it to the
effects of the crosswind. The small roll to the left, despite the pilot quickly applying corrective aileron, exacerbated this.
‘As a result, the aircraft moved closer to the left edge of the airstrip before bouncing again. It is highly likely that the continued left sideslip would have meant that a subsequent touchdown would have been off the grass surface, something which would have been evident to the pilot. This, together with the length of the airstrip, would have provided good cause to go around. The application of power and associated torque, corkscrew, gyroscopic and asymmetric effects, would have further increased the tendency for the aircraft to travel to the left. The ability to compensate with roll would have been limited, as the aircraft was close to the ground and over a cultivated surface. The direction of the wind would have led to the fuselage partially blocking the wind affecting the left wing, reducing its lift. The wheel contact marks (left in the ground) demonstrated that the aircraft was not climbing.
‘The go-around procedure calls for a compromise between power application and controllability. The pilot’s described technique differed from that in the aircraft manual. By leaving the gear down there would be an increase in drag, however the aircraft was too close to the ground to raise it safely. Equally, there is evidence that the pilot raised the flaps during the attempted go-around by 20° in an effort to reduce drag, but this would have resulted in a reduction of lift. The situation was compounded by the gear coming into contact with the cultivated ground which would have had a significant decelerating effect. The combined result was that the aircraft struggled to accelerate and remain airborne, and it veered approximately 30º to the left of the runway direction.’
Although both occupants were strapped in, and the Mustang’s cockpit remained largely intact, they were subject to considerable forces, sufficient to result in serious neck injuries. The AAIB draws attention to the fact that neither occupant was wearing appropriate clothing. The pilot was wearing cotton overalls and a composite helmet; the passenger was wearing normal clothing with no helmet. The CAA’S CAP 632 Flying Clothing notes: ‘Flying suits are the only practical garment for flying ex-military aircraft… [and] given the possible close proximity between fuel and the pilot, particularly in ex-military aircraft, wearing of fire-resistant flying suits such as those made by Nomex is very desirable and highly recommended.’ Nonetheless, the AAIB says that the passenger’s head injuries, likely to have been caused by hitting the inside of the canopy in the first ground impact prior to the canopy detaching, were severe and it is unlikely that the outcome would have been different had he been wearing a helmet. Another Mustang incident, but with a much happier outcome, occurred near Duxford on 9 July 2017. The aircraft was part of a warbirds formation that was at the end of its display and they separated to fly crosswind legs. As the Mustang’s pilot applied power, the engine stopped, restarted, ran at the selected power setting for a few seconds, then stopped again. The pilot transmitted a Pan call and prepared for a forced landing, during which the engine started and stopped several more times, allowing a gradual descent, before finally stopping.
With the aircraft in a tight downwind position at approximately 500ft and 150mph, the pilot selected 20° of flap, landing gear down, and started to turn onto base leg, but it became evident that the aircraft had insufficient energy and would not reach the runway. He retracted the landing gear, lowered 30° of flap and touched down in a cornfield at approximately 120mph. The Mustang remained upright and the pilot was uninjured. The cause of the engine stoppage was not established, but based upon the reported symptoms, its maintenance organisation suspected it to be carburettor-related. The aircraft has since been returned to an overhaul facility in the USA for investigation and repair. ‘The pilot’s recognition of the need to make a forced landing and configuring the aircraft in time for the landing ensured a successful outcome,’ comments the AAIB.
Ditched after unexplained power loss
Aircraft Type: Piper Cherokee 140 Date & Time: 30 March 2017 at 1450 Commander’s Flying Experience: CPL, 8,303 hours, 4,000 on type Last 90 days: 8 hours Last 28 days: 5 hours The aircraft was being used for a training flight to convert a PPL(A) holder to type, and this was the first flight of the day. It had been refuelled the previous day to 17 USG in each wing tank, sufficient for more than three hours of flight. The PPL(A), who was the handling pilot, selected the left tank for engine start and taxying before switching to the right tank for engine run-up and pre-takeoff checks, which were completed satisfactorily. As part of these checks the electric fuel pump was switched on and the fuel primer locked closed.
Takeoff and initial climb out were normal, with the engine running smoothly, but as the aircraft crossed the coast whilst climbing through 450ft the engine abruptly lost power, although the propeller continued to windmill. The PPL(A) passed control to the commander, who lowered the nose to maintain best glide speed and turned left towards the shoreline. During the descent he confirmed that the fuel was selected to the right tank, that the magneto switch was set to ‘Both’, the primer was locked closed and the electric fuel pump switch was on, then pumped the throttle to exercise the carburettor accelerator pump, but the engine did not respond.
As the engine continued to windmill without producing power, the commander realised that a ditching was inevitable, so he turned the aircraft to track parallel to the shoreline and made a Mayday transmission to Shoreham ATC. He selected two stages of flap and opened the cabin door. The sea was calm and the aircraft ditched ten metres from the shoreline, remaining upright so that both occupants were able to exit without difficulty and swim ashore. The aircraft subsequently sank, but was later recovered with no significant damage other than from seawater immersion.
After recovery the Cherokee was examined by the AAIB. There was no evidence of a fuel leak and no obstructions were found within the fuel system’s tanks, vents, filters, fuel lines, gascolator or electric and mechanical fuel pumps. The carburettor showed no defects, and the accelerator pump and mechanical fuel pump functioned normally. Inspection of the disassembled engine revealed no failures that would account for an abrupt and complete loss of engine power, although three of its camshaft lobes and the bearing faces of their respective tappet bodies were worn. However, the AAIB noted that while worn camshaft lobes would cause a loss of engine power output due to reduced inlet valve travel on all four cylinders and changes in valve timing, the engine ran smoothly for takeoff and climb prior to the loss of power.
The AAIB report concludes: ‘The absence of any engine rough-running immediately before the abrupt power loss indicates that the cause was probably not due to a fault with the dual-independent ignition systems. The commander described carrying out a carburettor icing check as part of the engine run-up checks prior to departure, with no carburettor ice detected. Given the ambient weather conditions (temperature 15ºc, dew point 10ºc) carburettor icing was more likely to form at low power
settings rather than the wide-open throttle setting used for takeoff. Therefore if carburettor icing had occurred, it would probably have been detected after the period of ground taxying to the runway holding point rather than during takeoff, which itself occurred shortly after carburettor heat had been applied as part of the carburettor icing check.’
Whilst the AAIB said that the power loss might have been caused either by contaminated fuel being drawn into the engine from the selected right fuel tank, following the accident the airport operator confirmed that the fuel sample from the batch used to refuel the Cherokee had passed the normal fuel quality examination, and that no other aircraft receiving fuel from the same batch had reported any fuel-related problems. The PPL(A) who was flying the aircraft had conducted the daily fuel drain check and found no contamination in the samples drained from fuel tanks, but due to seawater ingress into the fuel system after the aircraft had ditched it was not possible to determine whether any fuel contamination had occurred. The cause of the engine power loss was therefore not established.
Comanche ambushed by windshear
Aircraft Type: Piper Comanche 250 Date & Time: 21 July 2017 at 1055 Commander’s Flying Experience: FAA PPL, 1,257 hours, 388 on type Last 90 days: 1 hour Last 28 days: 1 hour The Comanche was on finals to Rwy 21 at Retford (Gamston) for a touch-and-go landing following a flight from Leeds. Forecast wind before departure had been 180°/14kt. The pilot selected an intermediate flap and increased airspeed slightly to allow for the crosswind. He reported that the approach had appeared to be “fairly stable”, with the landing gear down and flaps set, until at about 100ft agl he noticed that the airspeed had dropped by about 10kt. He recalled that the turbulence increased at this height and he felt the aircraft sink, but that he was able to correct for this. Then, at 20ft agl, he experienced further sink which he attempted to correct by increasing power, but the stall warner sounded and he responded by pushing forwards on the control column. However, he was unable to escape the down draughting air and the aircraft struck the runway nose-first. The nosewheel leg collapsed and the aircraft skidded along the runway with the propeller striking the surface and stopping the engine. The main landing gear collapsed before the aircraft slid to a halt. Neither occupant was injured.
The pilot reported that he had been landing at and taking off from Retford for 24 years and had previously experienced worse conditions of windshear there than those encountered during the accident flight. He commented that because the conditions did not seem bad during the early stages of the approach he was probably not mentally prepared for the downdraughts which affected the aircraft shortly before touchdown. Reviewing the accident conditions, he realised that when he experienced the downdraughts he had been in the lee of a long line of airport buildings and tall trees, so they might have created some rotor effect. He also noted that when the reported wind was passed to him by radio he was still some three miles from the airfield so, in hindsight, it might wise to have requested a further wind report when he was closer. After the accident, the airport’s rescue service recorded the surface wind at 150º/17kt.
The AAIB comments: ‘The International Civil Aviation Organisation has published a Manual on Low-level Windshear (ICAO Doc 9817) which states that buildings such as hangars and fuel storage tanks commonly cause low-level windshear, particularly at smaller aerodromes. It notes that even when such buildings are not especially tall, they tend to have large lateral dimensions and to be grouped together, thus presenting a wide and solid barrier to the prevailing surface wind flow. The wind flow is diverted around and over the buildings causing the surface wind to vary along the runway. Such horizontal wind shear, which is normally very localised, shallow and turbulent, is of particular concern to light aircraft operating into smaller aerodromes but has also been known to affect larger aircraft.’
Engine not performing well?
Aircraft Type: Taylor Monoplane Date & Time: 15 August 2016 at 1201 hrs Commander’s Flying Experience: Not established Witnesses reported that the aircraft’s Volkswagen engine seemed to lose power at about 100ft after takeoff from Manchester-Barton. The pilot tried to make a forced landing, turning left from the runway to avoid a built-up area, but the aircraft crashed in a wooded area just outside the airport’s perimeter fence and became lodged in the trees, trapping the pilot, who was freed by firefighters. He had suffered serious though not life-threatening injuries, but his health subsequently deteriorated after postoperative complications.
At the time of the accident the aircraft was undergoing an annual test flight for the renewal of the Certificate of Validity for its Permit to Fly. Prior to this flight the pilot had undertaken some maintenance on its engine which included removal of the four cylinder heads. This work had been overseen by a Light Aircraft Association inspector and ground runs had been conducted before he authorised the test flight.
The pilot had originally planned to conduct the test flight a few days prior to the accident, but had instead spent that day carrying out maintenance on the aircraft and conducting multiple ground runs and fast taxi tests. Airfield staff were aware that the pilot had been experiencing engine problems, but he had not informed, nor sought assistance from, his LAA inspector when it became apparent that the engine was not performing well.
‘It was not established what maintenance activity had recently been performed on the engine by the pilot, however there was some evidence that it might have involved accessing the carburettor fuel inlet,’ says the AAIB’S report. ‘The pilot was not able to assist [with our] enquiries. A post-accident examination of the engine and fuel system by an LAA Inspector did not reveal any anomalies which could have accounted for the loss of power during takeoff. Carburettor icing or fuel vaporisation, which can occur when using mogas [as the pilot was], could not be ruled out.’
At its September meeting the Airprox Board reviewed sixteen aircraft-to-aircraft and eighteen aircraft-to-drone incidents. Five of the aircraft-to-aircraft incidents were assessed as having a definite risk of collision (one Category A, four Category B),
Drones continue to feature significantly, with ten such encounters in Categories A and B. One Category A involved an Airbus A319 on an ILS approach to Gatwick’s Rwy 26 that was 6.3nm from touchdown when the first officer noticed a small black object close to the right side of the aircraft’s path and on a converging vector. At first he thought it was a bird, but it became apparent that it was a drone. The startle factor of the drone’s proximity nearly caused him to disconnect the autopilot for avoiding action. At its closest point the drone passed between the Airbus’s right wingtip and its fuselage. The crew reported the encounter to ATC and Gatwick police attended once the aircraft was on stand. The drone was described as “very large, certainly not a toy”, with an estimated diameter of about 1m, and with four rotors. The A320 captain’s opinion was that a larger aircraft might not have missed it, and that it had put 130 lives at risk.
In another incident a Cessna 177 in straight-and-level flight at 2,000ft had just received MATZ penetration approval from RAF Coningsby when a drone was seen passing in the opposite direction some 50-100ft below its left wing. No avoidance action could be taken, as it was not spotted until the pilot became aware of it in his peripheral vision alongside his aircraft. It was close enough for him to make out four rotors encased in a black/white body. An Airprox was immediately reported on frequency to Coningsby. The drone appeared to be turning left, so it is likely that its operator had seen the Cessna and was taking avoiding action.
For full details go to airproxboard.org.uk.
The ditched Cherokee was recovered with no significant damage
The unfortunate Mustang flying in happier times