An Aeroflot-Nord pilot flying a Western-built aircraft – a Boeing 737-500 crashed on approach to Perm, Russia, last year because of disorientation. The official report says the accident was at least partly caused by the fact that the Western and Russian artificial horizons (AH) – alternatively known as attitude director indicators (ADI) – work on a completely different psychology.
This captain had spent most of his flying life using the Russian model and crashed using the Western one. Most psychologists argue that the Russian model is more effective. We’ll look at why in a moment.
For a pilot properly trained on either AH, they are equally good at providing him/her with clear information about the aircraft’s attitude relative to the real horizon when the latter is obscured by night or cloud.
But what if the pilot was brought up for years using the Russian format, then changes to the Western one? Normally no problem, but disorientation almost always begins with some form of distraction away from the instruments, and if the pilot looks back to the AH and sees an attitude he is not expecting, that’s when the trouble can start.
Have a look at the picture above of the two AH types, both showing that the aircraft is in a 40deg bank turn to the right.
We’ll start with the Western model, the one on the left, because its visual logic is easier to appreciate. The horizon bar of this AH always mimics where the real horizon is. What the pilot flying sees, as he rolls his aeroplane (and himself, and the whole instrument panel) into a turn to the right, is that the aircraft symbol, which is fixed to the instrument and thus to the real aircraft, stays in the same attitude relative to everything around him in the cockpit. Another way of considering what the pilot sees in this situation, is that the component that moves relative to the pilot (and relative to the cockpit and to the aeroplane symbol) is the horizon bar, which begins tilting to the left. If the real horizon is also visible, that’s what it is doing too.
This is where the psychology comes in. It’s to do with what appears to be moving relative to the pilot, and what appears stationary relative to the pilot. We’ll come back to that psychology after looking at the logic of the Russian AH.
As the pilot flying in, say, a Tupolev Tu-154 rolls into his 40deg bank right turn, he sees the aeroplane symbol moving into a right bank relative to himself and everything around him in the cockpit. That is a very compelling image: the pilot has commanded a right roll, and the aeroplane symbol rolls right relative to him. The potential psychological confusion here occurs if the real horizon is also visible, because the AH horizon bar is clearly not aligned with the real world’s horizon; in the Russian version the horizon bar is the component that is fixed to the aeroplane.
The psychology of flying the Russian AH involves imagining the aircraft symbol as a remotely controlled aircraft that you are flying, and you can make it do what you want it to – but you are watching it from the outside. It is more like a computer game. The instrument tells the complete truth about the aeroplane symbol’s relationship with the AH horizon bar, so all the pilot has to do to recover to real straight and level flight is to fly the aircraft symbol onto the horizon bar.
When pilots are led astray by the Western AH, the problem usually starts because the pilot is stressed by high workload or, because of distraction, he has already become disorientated.
Imagine that the pilot with a Western AH has commanded a roll to the right, intending to stop at 30deg right bank, gets momentarily distracted, then looks back at the AH and sees the bank passing through 40deg and continuing to roll right. In a moment of panic, if the pilot latches visually onto the AH component that is moving relative to him – the horizon bar - he may try to “fly” the horizon bar instead of the aeroplane symbol. Think of it: relative to him, the horizon bar is rotating left, so if he sees that as the object to be controlled, the result will be control inputs that steepen the roll to the right, and the real aeroplane may enter a spiral dive. The AH will be telling the pilot the truth about the aircraft’s attitude, but while he persists in “flying” the roll bar the psychological result can be complete confusion about what is happening.
The advantage of the Russian model is that the pilot is less likely to end up trying to “fly” the horizon bar because the aircraft symbol is more dynamic relative to the pilot’s field of view. It is more obviously the component that demands to be controlled.
You don’t have to have learned on a Russian AH to misinterpret a Western ADI. Disorientation and stress can lead a western-trained pilot to try to “fly” the horizon bar instead of the aircraft symbol. It’s what probably happened, according to the US National Transportation Safety Board, in the case of John F. Kennedy Jr, in 1999. He was flying a private Piper Saratoga at night when he lost control.
In the video mockup of what happened to the Aeroflot-Nord 737, the fatal banked turn was to the left, but the pilot’s action followed the classic pattern whereby the roll continued until it became steep and eventually inverted, and the aircraft’s nose dropped as a result.