Russian investigators have detailed the serious damage inflicted on a Nordwind Airbus A321 by a hard landing at Antalya, after which the crew executed a go-around despite the aircraft’s being crippled.
Over 100 failures were recorded by the ACARS communications system after the twinjet – arriving from Moscow on 10 January 2020 – pitched nose-down just before landing on runway 36C, and struck the runway nose-gear first.
The impact resulted in a failure of two inertial reference systems (IRS), as well as one of the flight augmentation computers, and this led to a loss of heading, pitch, roll and speed information on the captain’s side.
Russia’s Interstate Aviation Committee says this probably resulted from damage to the air-data reference units in the avionics bay during the nose-gear strike.
Although a third IRS – providing data to the first officer’s side – was still functioning, its pitch information “jumped” at the moment of impact and stayed 3° out of alignment with the information on the standby instrument.
Failure of two IRSs would typically switch the aircraft to ‘alternate’ law, because the elevator aileron computer would be unable to carry out calculations for ‘normal’ law.
But the inquiry found the aircraft shifted instead to ‘direct’ flight-control law, and the flight-director command bars were disabled.
Although moving to ‘alternate’ law while the landing-gear is down would result in a switch to ‘direct’ law, the nose-gear impact had affected the sensor and triggered a false gear-up signal. The inquiry found the change to ‘direct’ law was actually caused by the elevator aileron computer’s concluding that the remaining functioning IRS was unreliable, after a vertical load check, and rejecting all three of them.
As the crew advanced the thrust levers to go-around power and the aircraft started to climb, the landing-gear remained extended – the pilots claimed the gear lever had jammed. The inquiry says that the pitot-static and angle-of-attack sensors on the nose were not damaged, but the “significant wrinkling” of fuselage skin in the vicinity could have affected their accuracy.
The crew received an alert for smoke in the avionics bay during the climb. Although the pilots put on oxygen masks, the inquiry indicates that mist from a spraying hydraulic fluid leak, rather than a fire, had triggered the smoke detector.
A321s have three hydraulic circuits – yellow, green and blue – which normally operate with a pressure of 3,000psi.
Pressure in the yellow circuit fell to 1,270psi, low enough to trigger an alarm, while that for the green circuit declined to 2,100psi. Fluctuations in pressure caused some hydraulically-powered systems to alternate between being functional and non-functional.
The remaining IRS failed while the aircraft was flying level at 3,450ft, probably due to distorted navigation calculations from the dislodging of the air-data reference unit. References to attitude and speed disappeared from the first officer’s displays.
The captain, operating the aircraft with visual guidance and the standby instruments, opted to make a low pass of the airport for a landing-gear inspection.
About 90s after this decision, the yellow hydraulic system suddenly lost pressure and failed completely. The loss of fluid spray, combined with the crew’s ventilation actions, meant the smoke warning stopped.
But the failed yellow hydraulic circuit, and the insufficient pressure in the green circuit, meant the flaps remained at their 30° setting after the crew tried to select full flap. The slats, which used the blue circuit, did extend.
The low pass over the airport confirmed that the nose-gear and main landing-gear were extended, and the crew prepared to conduct a second approach.
Further hydraulic problems emerged when the green circuit’s reservoir started to overheat. Differential pressure between the yellow and green circuits caused the aircraft’s power-transfer unit to function continuously, instead of being inhibited by a nose-gear signal. The signal was absent due to nose-gear impact damage.
The inquiry says that two brief “jumps” in the yellow circuit pressure occurred which, while brief, were enough to allow the flaps to extend fully. The yellow circuit then dropped to zero pressure, followed by a similar loss of the green circuit a few seconds later when the first officer, in response to the overheat warning, turned off the green hydraulic pump. Loss of the green circuit meant a number of systems became unavailable, and the crew declared an emergency.
Investigators state that a total of 103 system failures were recorded by ACARS in the aftermath of the runway impact.
After being vectored to runway 36C the crew carried out the approach in ‘direct’ law, with limited control of the configuration or the horizontal stabiliser, most of the spoilers unavailable, and without reverse thrust.
But despite deviations from the glideslope, the twinjet touched down safely at 125kt with a 1.23g impact and braked to a halt on the runway. None of the seven crew members, the only occupants, was injured.