Continental 3407 is just the most recent example of why turboprop passenger aircraft are unable to safely fly during icing conditions.
The American Eagle ATR 72 in Indiana, the Embraer 120 in Michigan, and some fifty Cessna Caravan crashes, all demonstrate the need to reassess whether turboprop aircraft, all equipped with long outdated and discredited deicing boots for ice protection, should be permitted to be dispatched when the weather looks like ice might be experienced, as in winter clouds.
All of these aircraft have wings that have high aspect ratio, meaning they are long and thin from front to back. All of them have tiny horizontal tails and use deicing technology rather than anti-icing technology to be certified for flight in known icing conditions. In short, the manufacturers of each of these aircraft convinced their certifying authorities to approve the aircraft to fly in weather conditions conducive to the formation of airframe icing because their deicing boots (inflatable rubber boots on the leading edges) could discharge enough ice to allow them to continue flying safely.
The reality is that large airplane manufacturers gave up deicing boots fifty years ago because they knew that they don’t work effectively. By design, deicing boots assume that ice will be allowed to collect on the wings and tail of the airplane and then be shed when the boots are inflated. The flaw in that theory is that most boots do not shed all the ice; in fact, residual ice continues to build, further impeding their performance until they just don’t work well at all. Additionally, deicing boots are usually limited to five percent of chord – the distance from front to back of the wing – so runback icing which collects beyond the boot coverage is unaffected by boot inflation.
Why do turboprops use boots? Simple. Turboprop manufacturers have been slow to utilize state-of-the-art anti-icing technology like heating the leading edges of the wings or extruding glycol from the leading edges (which then runs back and keeps the wings free of ice). To heat the leading edge, one must extract heat from the engines, compromising power and increasing fuel consumption. Also, many turboprop engines just don’t have enough bleed air to use for this purpose. The TKS, or glycol system, requires plumbing, a wire mesh leading edge that needs frequent cleaning as well as glycol, which increases the aircraft’s weight.
This age-old problem is what took Continental 3407 down. The weather from a warm front that had overridden cold air at the surface consisted of rain that was falling into the cold air and becoming SLD, super-cooled large droplets. These drops, still liquid but below freezing, immediately turn to ice when they touch a cold surface like an aircraft wing or tail and then run back past the boot coverage, contaminating the airfoil and destroying its ability to create lift. What pilots are not told is that no aircraft is certified to fly in freezing rain or drizzle so if they are dispatched in such weather, they are test pilots.
In most instances the exposure to SLD is brief but for Continental 3407 it wasn’t brief enough. It encountered freezing rain, and as a result, accumulations of ice on the wings and windshield, a condition which was discussed by the crew. They activated the deicing equipment and fully expected it to take care of the encounter. What they didn’t know was that the aircraft was never certified or equipped to handle freezing rain or drizzle and that ice was collecting on the tail, aft of the boot coverage, which they could not see.
As they descended for their instrument approach, the crew was unaware that another danger awaited, the extension of their wing flaps. With the flaps up, the tail was barely able to exert a down force on the aircraft sufficient to keep the nose where the crew directed it. Once the flaps and landing gear were extended, however, the tail would have to counteract the tendency toward a down nose pitch caused by those extensions. It simply could not do that as a result of the ice contamination. Just as soon as the flaps were extended, the load requirement on the tail increased, it stalled, quit flying, and the nose of the aircraft pitched suddenly down. Ice on the wings aft of the boots rendered the ailerons less effective in controlling roll, and the aircraft rolled violently as well. The crew immediately attempted to retract the landing gear and flaps, just as they were taught to do, but they had insufficient altitude to recover and insufficient elevator authority left in the tail to arrest the steep pitch down. The aircraft struck the ground in a near vertical attitude, perhaps flattened at the last moment due to the flight deck crew’s quick thinking to retract the flaps. However, it was insufficient to arrest the descent and all aboard were killed instantly.
This accident was a duplicate of the ATR-72 and Embraer accidents of years ago and similar to many of the Caravan crashes as well.
What do all these aircraft have in common? They all have deicing boots instead of anti-icing equipment. While many of the Caravan crashes have occurred in very benign icing environments, deicing boots have repeatedly shown their inability to safely see an aircraft through winter flying conditions that can be anticipated. The accident rate and its toll on human lives are simply unacceptable. The manufacturer of the Dash 8 Q400 that was Continental 3407 knew better than anyone that boots were wrong for aircraft of this size and use. It doesn’t use boots on any other aircraft it manufactures and for good reason.
Likewise Continental knew, or should have known, that it was dispatching Flight 3407 into freezing rain and drizzle and its aircraft was not certified for that flight condition. More likely than not, Colgan Airways, the Continental contract carrier operating the flight, was likely told the Dash 8 could operate in all winter weather that could be anticipated. In fact, it simply could not.
The NTSB will investigate and the Canadian Transportation Safety Board will participate. The result will be long and expensive studies into icing once again. That’s what governments do – they study, study and re-study but they do not fix the problem. The fix is simple. Restrict the flight conditions under which turboprops can be dispatched. They are already restricted, but obviously the airlines don’t yet get it. Then re-equip all turboprops used for passenger transportation with more effective anti-icing equipment. Lastly, prohibit the extension of flaps for any landing where accumulations of ice are suspected on the aircraft. Turboprops, and especially booted turboprops, have no place any more in the transportation of people. Their time has come and gone. Good riddance!
Arthur Alan Wolk
February 13, 2009