Early information released by the US National Transportation Safety Board about the Colgan Air Bombardier Dash 8 Q400 that crashed on approach to Buffalo on 12 February made it look as if the agency had more or less decided that icing was the cause.
This is definitely no longer so. Icing now looks like just one factor – maybe not even the most important one - in a more complex series of events and influences. All the information available at present indicates that the icing that night was at worst moderate and perhaps only slight, neither of which would have been a problem for the Q400′s icing management systems if they were fully serviceable. They had certainly been selected.
Although circumstantial evidence (classic icing conditions prevailed, the stick-pusher kicked in, the aircraft pitched up rapidly), suggests icing-caused aircraft behaviour, there are still some alternative possibilities that are unresearched. These include the possibility of an aft centre of gravity exacerbated by other factors. The other factors could include the forward movement of the centre of lift on the wings caused by a combination of light ice contamination followed by the deployment of the flaps. Combinations like these cannot be ruled out until the investigation has had time to analyse the accident data they are still collecting.
For that reason I’m disappointed that the NTSB has handed the press a piece of information that has been interpreted by much of the media as meaning that the aircraft crashed because the aircraft was on autopilot when the crew should, in icing conditions, have been flying it manually. That is horrifyingly misleading at this stage.
One thing that this investigation will definitely NOT conclude, when the final report is published, is that the aircraft crashed because it was on autopilot when it went out of control.
Since the outcome of extensive research by the FAA on icing as it affects turboprops following the investigation into the October 1994 crash of an American Eagle ATR72 twin turboprop at Roselawn, Indiana, crews have been advised (not required) to trip out the autopilot in known icing conditions so they are more aware of abnormal or unusual control forces building up.
That is basically a sound piece of advice but it does not guarantee that, just because the pilots are manually controlling the aircraft, they would necessarily be able to prevent the departure from controlled flight of an aircraft whose de-icing systems are being overwhelmed by the prevailing conditions.
Let’s also just have a generic look at what icing protection turboprop aircraft have, especially those with only one or two engines. The first point is that it is not as comprehensive and powerful as the systems jets have, because turboprops are so efficient at converting fuel into power to drive the aircraft forward that they have a smaller proportion of engine power left over to generate electricity or provide hot bleed air – the two most commonly used forms of energy used for anti-icing.
The large surfaces that need icing protection on turboprops are the leading edges of the wings, tailplane, and fin. While smaller areas like engine intakes, propeller roots and spinners can be (and some are) anti-iced by electrical heating, achieving that over the large areas would be beyond the capability of the aircraft’s electrical generation system.
So the system most turboprops - including the Dash 8 and ATR series - use is the rubber de-icing “boot”. These black “boots” that wrap around the leading edges of all the lift-producing surfaces work by employing engine bleed air as economically as possible. The de-icing system provides pulses of pneumatic energy to cause the boots to inflate slightly, then deflate, then re-inflate and so on. This is intended to break off ice that has already formed on the leading edges.
Yes, you did read that correctly: the ice has to form first and then be broken off, and this is called de-icing, not anti-icing.
If the system is operated before ice starts to build up, there is a risk that, when buildup begins, the pulsed inflations of the boot will cause a gap to form between the ice buildup and the deflated boot, so when it re-inflates it has little effect on the ice that can quickly wrap around the entire leading edge to points beyond the boot itself.
So although the system works well if used at the right time, it can have reduced effectiveness if used at the wrong time. Knowing the difference between the right time and the wrong time is difficult because we are talking about weather here, and no two sets of conditions are ever the same.
In the end, turboprop pilots have to take entry into icing conditions even more seriously – in fact much more seriously - than jet pilots do. And one of the things they need to be able to make good decisions is good forecasts and actual weather reports. Avoiding icing conditions if you are unsure of their intensity is the best policy, but if you trust the information you are given and it turns out to be on the optimistic side of inaccurate, you are in trouble.
Icing is, perhaps above all, the weather condition that remains the most unpredictable and uncontrollable, and the airborne systems for dealing with it are still basically the same as they were in the 1950s.