Emergency

Pilots train extensively on detecting and managing faults with the aircraft's systems, and they are evaluated annually in a simulator. Detecting faults early requires continuous monitoring of the instruments. If one receives an indication that something is abnormal, it is important to attempt to understand what the problem is before concluding what must be done, so as not to end up making matters worse.

When a fault situation arises during flight, the pilots must identify the cause and take the necessary measures to reduce risk, as well as assess whether the flight can continue as planned or must be aborted. Some situations arise from acute problems, while in other cases the severity is not immediately apparent. An engine failure is easy to detect, and the pilots are in no doubt about what they must do. If, on the other hand, one experiences a reduction in engine performance, or for instance an indication of fluctuating oil pressure, it may be the beginning of a problem that develops into something serious.

Vibrations

All pilots have learnt that if structural damage to the aircraft occurs, speed must be reduced as soon as possible to minimise the load on the aircraft structure. If vibrations arise in engines or control surfaces, it is important to determine whether the vibrations are related to specific speeds or engine settings. If the vibrations do not quickly subside, most pilots will reduce speed considerably and find a suitable landing site.

Performance Loss

A performance reduction in one engine does not necessarily mean there is a serious problem, but it must be investigated. A reduction in the performance of the left engine will cause the aircraft to begin turning to the left, as the thrust is no longer symmetrical. The autopilot will automatically correct the turn, but the pilots will be able to detect that something is wrong because the speed decreases and the aircraft begins to fly uncoordinated. The cause of the speed reduction would in that case be that the total thrust is reduced, and that the use of control surfaces to correct the heading causes increased drag.

Priorities

«Aviate, Navigate, Communicate» is the order of priorities for pilots. The most important thing is to maintain control of the aircraft (aviate). Once control is established, one can formulate a plan for where to fly and navigate in the correct direction. At the bottom of the list is communicating with the air traffic service.

It may sound illogical not to send a distress call first, but in practice there is very little an air traffic controller can help with if the pilot is unable to control the aircraft or navigate to an airport. If no distress call has been received in connection with an aircraft accident, it means either that events unfolded very rapidly, or that the pilots chose not to send a distress call because they were occupied with more important tasks.

The Lead-Up to the Accident

The accident investigation board has based its conclusion on the premise that the accident came suddenly and unexpectedly to the pilots, while simultaneously claiming that the cause was vibrations that had intensified over time. The problems are said to have been impossible to notice or to have produced any indication on the instruments, otherwise the crew would have detected them on this and previous flights. The main argumentation for this is that the aircraft appeared to be flying normally—something previous chapters have shown was not the case—and that no distress call was transmitted.

A far more probable scenario is that the crew detected that the aircraft was developing an unwanted turn to the left, but misunderstood the cause and believed it was related to the left engine rather than the rudder. By the time they realised there was damage to the aircraft's control surfaces, the damage had already developed to catastrophic proportions.

Without recordings from a Cockpit Voice Recorder, it is very difficult to establish with certainty what the pilots were thinking and doing prior to the accident. When one combines data from the Flight Data Recorder (FDR) and radar plots with knowledge of how the crew was trained, it is unlikely that the pilots of LN-PAA were unaware of problems with the aircraft right up until the final sequences, as the accident investigation board has assumed.

Pilots use checklists during the various phases of flight to ensure that nothing important is overlooked or forgotten. In addition to the normal checklists, pilots also have emergency checklists, which describe each step to be taken in a given emergency situation—for example, checklists for engine fire and engine failure.

Without cockpit audio recordings, it is difficult to know which checklists the crew of LN-PAA used. Instead, one must rely on knowledge of what pilots have been trained to do, as well as the context of the particular flight, in order to form a picture of the crew's pattern of actions.

Speed Loss After "Top of Climb"

After the aircraft had reached "top of climb" and built up speed, the pilots set the engines to "cruise power." Despite the fact that cruise speed should have stabilised, it gradually declined. During this phase of the flight, the airspeed indicator and the Turbine Inlet Temperature (TIT) are two essential instruments. It is highly probable that the crew quickly perceived that the aircraft was not stabilising at the expected cruise speed, but was gradually losing airspeed. Based on an expected cruise speed of 210 knots indicated airspeed, this means the crew became aware of the problem at approximately 29 minutes FDR time.

Autopilot and Troubleshooting

If problems arise with the control surfaces, the most important action would be to reduce speed and then disengage the autopilot in order to feel the movements of the control surfaces and the aircraft through the controls. An autopilot can mask the effects of structural damage—vibrations and tendencies towards rotation about one of the aircraft's axes become difficult to detect when the autopilot is engaged. The autopilot has no control over the engines, and if one receives an indication of an engine problem, it would not necessarily be natural to disengage the autopilot immediately.

Given that the crew had recently adjusted the engines, and that the result was declining airspeed, it would be natural to suspect that the problem was engine-related. It is reasonable to assume that the crew attempted to adjust the engines to compensate for the speed loss, but this is difficult to read directly from the FDR data. The only thing one can rely on is that the engine settings, at some point before the accident, were set such that the Turbine Inlet Temperature was 850–853 °C instead of 847 °C, which would have been the expected temperature for "cruise power."

If the autopilot is using the control surfaces to correct for forces about the vertical axis (yaw), this will produce a deflection on the slip indicator in the aircraft. After having troubleshot the engine instruments, it would be natural to disengage the autopilot to see whether it was the cause of the reduced speed. If the autopilot was fighting against a tendency to turn left—which would have been the consequence of a performance reduction in the left engine or increased drag on the left side—the aircraft would have immediately begun to turn left when the autopilot was disengaged.

The Three Heading Deviations

The first of the three heading deviations to the left occurs approximately 1 minute and 30 seconds after the aircraft's speed dropped below the expected cruise speed. It seems probable that the heading deviation resulted from the autopilot being disengaged, as this is a natural step in the troubleshooting process. Since the speed did not drop dramatically to well below 200 knots, it seems unlikely that the crew suspected structural damage. If at this stage they still suspected a problem with the left engine, it seems logical that the crew corrected the heading deviation with aileron input to the opposite side.

It is impossible to know in hindsight what the crew was thinking, what sounds they heard, what movements in the aircraft they felt, and how they reasoned together to understand what was wrong. What seems entirely obvious is that they must have understood they had a problem, and that they were working to manage the situation. The fact that the crew did not react by reducing engine power and airspeed can be interpreted as meaning that they did not understand the aircraft had sustained structural damage. When the first indications arose, there was no way for the pilots to understand how little time they had before the situation became catastrophic.

The Flight Engineer in the Cockpit

Since the accident investigation board's hypothesis is that the crew did not know the aircraft had problems until it was too late, they have also not discussed what impact it had that there was a flight mechanic together with the pilots in the cockpit.

That the problem arose just after the engines had been adjusted from "climb power" down to "cruise power," and that declining airspeed was the most visible indicator, makes it natural to suspect that the problem was engine-related. When one additionally has a technician with in-depth knowledge of the engines present, this may have reinforced the focus on the problem being engine-related.

Given that the aircraft was approaching Denmark, where there were several alternative landing sites, and the pilots apparently had control of the aircraft, it is understandable that they attempted to maintain altitude and speed—given the assumption that the left engine was the root of the problems.