An Airbus A320 airliner of the Smartlynx airline made emergency landing in Tallinn Airport last February. The medium-size jet received such damage on the impact that it was declared unfit for further service. Now, a year and a half later, the experts of the Estonian Safety Investigation Bureau have determined the cause of the accident.
Training flight over Tallinn becomes survival exercise
It turned out to have been a training flight to the manufacturer Airbus as well as the trainee pilots, since they had to upgrade their aircraft after the causes had been determined.
The Smartlynx pilot school has borrowed a 150-seat Airbus A320 from the parent firm for training. There are seven people on board: four trainees, the instructor, the safety pilot and an inspector of the Civil Aviation Authority. As usual, the trainees pilot the aircraft, practicing touch-and-go. This means that they land the aircraft, touch the runway for a moment and take off again. The weather is cold, cloudy and calm.
At 17.05 the aircraft is approaching the runway in the Lagedi direction. However, after touching the ground the aircraft no longer takes off but keeps speeding towards the end of the runway. “Rotate,” says the instructor to the trainee. “I am rotating,” the latter answers, pulling the control stick towards him to make the aircraft lift its nose. But the aircraft does not respond. It keeps flying low over the runway and gathers speed, since the engines prepared for take-off are at high power.
“Taking over,” says the instructor and takes hold of the second set of controls. The trainee relinquishes control. Now the instructor learns that the trainee had not been dreaming and the aircraft indeed fails to respond. No one understands why.
The instructor keeps pulling the control stick and holds it for 14 seconds. Some 950 meters remain until the end of the runway as the speed becomes sufficient for take-off. Although the aircraft is very slow at gathering altitude, the instructor has to operate according to regular take-off routine: retract undercarriage and lift wing flaps. But since the retraction of the flaps reduces the wing area, the lifting power declines accordingly and the aircraft begins to lose altitude over the runway. It hits the tarmac from the altitude of 15 meters at the speed of 352 kilometers per hour. The impact smashes undercarriage covers and damages the engines.
Now the situation becomes even more confusing for those on board, since the impact tilts the aircraft nose skywards and with the engines at high power, the aircraft begins to rise at a steep angle. Both airspeed and take-off angle are higher than at regular take-off. While the regular passenger aircraft gathers altitude at the rate of some 460 meters per minute, the present rate is 1,800 meters per minute.
“Manual pitch trim only”, ENG 2 FIRE”, “ELEV FAULT” are only some of the announcements of the onboard computer. Audio alarms sound on the flight deck, warning the crew of malfunction. The instructor flying the aircraft has understood by now that the control stick has no effect on its attitude. Lateral control has fortunately been retained. The only option for regulating altitude is the mechanical one via the trim wheel. This disc next to the pilot’s seat is linked via wires throughout the aircraft to the stabilizer in the tail. This in turn is linked to the elevator.
The instructor tries to restore control. He turns the trim wheel to lower the nose of the aircraft towards the ground and also reduces engine power. But the effect is more than expected – the aircraft assumes nose-down attitude and begins a headlong descent from the altitude of 485 meters.
The instructor turns the trim wheel in the opposite direction to bring the aircraft out of the dive. As less than 200 meters separate them from the ground, the warning buzzers of the flight deck are accompanied by voce warnings: “sink rate”, “pull up”, “terrain, terrain, too low, terrain”.
The instructor finally manages to take control the aircraft. Only one minute and 18 seconds have passed since the beginning of the whole sequence of events. Still no one understands what has happened and why.
“Are the engines working?” the instructor asks. “Engine 2 on fire,” says the safety pilot, who has learned this from the error message on screen. The aircraft flies stable at 396 meters over the village of Maardu.
“Mayday, mayday, mayday,” the instructor tells the air traffic control. The safety pilot, who is reading error messages from the onboard computer’s screen, takes over communicating with the control tower. “Mayday, mayday, mayday. We have control problems,” he says.
What they need to do now is to bring the aircraft to the ground as soon as possible. One engine is already burning and control systems have failed – anything could happen next. The safety pilot recommends making a right turn and landing visually on the runway, and reports this to the tower. The message reaches the tower one minute and 53 seconds after the first problems were encountered.
The trainee, who was making a regular training flight a couple of minutes ago, is still sitting in the seat of the co-pilot. The instructor orders that the safety pilot and the trainee switch places. The trainee and the CAA inspector, who was also present on the flight deck, return to the passenger compartment where the other three trainees are sitting.
Now as the landing plan has been made, the safety pilot begins to read the operating manual check list for engine fire. The procedure calls for turning off the engine. But the instructor decides to ignore the manual. “If I am flying manually I prefer to land with engines running,” he explains.
The seconds later the instructor calls for lowering the undercarriage. He does not know that the wheels have been down since the aircraft first hit the runway.
The runway is already in sight as engine number two, which used to burn, suddenly stops. A long fire alarm follows. Some twenty second later the remaining engine also stops.
Everything becomes silent. Power is gone from the control system, indicators and navigation gear together with the lights on the flight deck and in the passenger compartment. All the screen displays is the most general information: altitude, speed, horizon. Now there is little left for the pilots to do. The aircraft is simply gliding towards the ground. At 17.10 it hits the ground 150 meters away from the runway. Tires burst and the aircraft rolls on its wheel rims. One of the undercarriage covers was later fund near Tuulevälja. All aboard survived and received no serious injuries. The safety pilot and the CAA inspector were from Estonia, the others were foreign citizens.
“There was barely enough speed and altitude. The instructor and the safety pilot were lucky to land the aircraft,” said Karl-Eerik Unt, air incidents investigation expert of the Safety Investigation Bureau. It was his job to find out why a routine training flight transformed into an accident.
A malfunction of an aircraft usually requires that several factors must coincide. This is what happened this time.
Unt learned about the accident immediately after the crash landing. Upon reaching the airport the first task was to gather evidence and clear the runway as soon as possible. Since the accident damaged the undercarriage, tires and wheels of the aircraft, changing the wheels was the first thing to do.
The evening was cold and there was a long night ahead; therefore a tent was set up near the runway for the workers to rest and warm up. Flight recorders were removed from the aircraft – the voice recordings of the flight deck as well as the “black boxes”.
After the wheels had been replaced, it was possible to winch the aircraft to a hardstand near the runway. All these urgent tasks were completed only by early morning.
But the investigators could not afford a rest after the long night. A news conference as held in the airport in a couple of hours. There was not much information to release, since the cause of the accident was still a mystery.
The same applied to the people on board, who were first interviewed after the press conference. Those on flight deck said that everything had been almost as usual before it turned serious – with the exception of one detail. Every time after the aircraft took off after touch-and-go, the onboard computer issued an error message “ELAC pitch fault”. This meant that the main computer had crashed and control had been handed over to secondary computers. The pilots rebooted the computer after the error message according to the operating manual. The error was cleared and the main computer turned on again.
Thus the investigators had two pieces of information in the morning after the accident: the onboard computer repeatedly issued the same error message and eventually the elevator, controlled by the same computer, failed to respond.
The investigator packed up the flight recorders and traveled to France. It was proved there that the error concerned the computers. But it also emerged that the main computer had not been rebooted one time after the error message, which meant that the control systems were run by the secondary computer before the final landing.
Having returned to Estonia, the investigator addressed the aircraft once again. Electric and hydraulic systems were turned on. A series of tests was launched in order to recreate the same error messages witnessed by the pilots during their training flight. The investigators managed to produce the same error message during the first tests. It seemed that the cause becomes clearer, but then the error messages ceased.
Air accident investigation can initially only guess at the causes and test a number of options. Therefore the electric power system was dismantled and the sensors were tested to fund any flaws. Nothing was wrong. Maybe it was the temperature? The elevator control system was cooled down with liquid nitrogen and dry ice to test the idea.
A new test was made and the error message was back. The suspicion was proven and repeated tests showed that the error message was shown when the temperature of the elevator mechanism remained below 6-9 degrees Centigrade. The day of the accident had been cold as well. The puzzle pieces were coming together. The gear was removed from the aircraft and sent to a lab.
It was necessary to ascertain, which part of the gear was flawed. It eventually emerged that the flaw was with the actuator, which did not permit the computers sense its engaging and the confusion resulted in computer crash. Oil samples were takes from the actuator and its all components were analyzed.
It emerged that the actuator gearbox contained wrong type of oil. “Wrong” meant that the oil was of higher viscosity, which increased the slipping of the actuator. Since the actuator was slipping and the computers could not sense its engagement, the computer crashed.
The investigation took a year and a half and comprised numerous tests, analyses and work groups. It swerves as an important lesson for Airbus, which needed to upgrade the computer system of its aircraft.
However, it remained unclear, who and when had added the oil with excessive viscosity to the actuator. It had been last maintained in the USA. The maintenance documents did not show the use of wrong oil. Yet it should be reminded that the firm involved was not an official organization for the maintenance of the Airbus A320 elevator gear.
As it appears, the accident occurred due to combined effect of several malfunctions. The conclusions of the investigation report do not point out a concrete culprit, since there were multiple factors involved.
It cannot be denied, however, that it was a rather fortunate accident. Investigator Unt said that worse consequences were avoided by the extensive experience and skillful reaction of the captain – the instructor pilot. For example, his decision not to turn off the burning engine contrary to the standard procedure was certainly correct in this case.
The aircraft received serious damage on impact and was no longer considered airworthy. The hull was purchased by the German army, which will use it as a training aid at the special forces base.