Considering prop rotational directions and the resulting asymmetry of lift and drag due to icing buildup under helical prop slipstream influence
BUFFALO, New York (USA) - In the minutes before a turboprop plane plunged to the earth killing all 49 people aboard and one person on the ground, the pilot and crew were recorded discussing "significant ice build-up" on the plane's windshield and the leading edge of the wings, federal investigators said today.
The "black boxes" recovered from the burning remains of Continental Express Flight 3407 also indicated that the de-icing button in the cockpit had been in the "on" position.
Shortly after that conversation, Capt. Marvin Renslow deployed the plane's landing gears and the wing flaps to slow down the plane in preparation for landing.
"Severe pitch and roll [began] within seconds" of the flaps being deployed, said Steven Chealander, spokesman for the National Transportation Safety Board. Chealander said that means the plane's nose bucked up and down while the wing's dipped and rose violently.
The plane plummeted to the earth so rapidly that Renslow and his crew never had time to radio a mayday alert.
Re the revelations above, two theories are worth pursuing (but they're not mutually exclusive):
The asymm icing scenario (as induced spanwise by same-direction prop rotation on each engine
a. firstly, that the flap extension suddenly exacerbated the differential between the left and right wing's icing asymmetry (i.e. by rapidly changing each wing's stalling angle-of-attack - and the L&R tailplane's also). Think and concede "effective wing leading-edge boot de-icing" but, in freezing rain, areas further back on each wing's chord would contain varying (and significant) thicknesses of spanwise-ridged ice accretions on particularly the upper (but also on lower) wing surfaces. At lower (approach) speeds the difference between the L&R wing's elevated stalling speeds (at the higher AoA) would be even more critical. With flap extension, those ice ridges would have an increased asymmetry effect upon the power of a differential frieze aileron to impose (or oppose) roll. Additionally (and more lethally) however, there'd be an increased likelihood that a large aileron deflection (turning onto the localizer) would be more likely to stall the upgoing wing and induce autorotation. The slipstream effect of added engine power at lower speed (due to gear/flap extension drag) would also accentuate the differing lift and drag coefficients between the two wings (and across the horizontal stabilizer).
The TailPlane Stall Scenario
b. Secondly, in respect of tailplane stall due to ice build-up on the undersurface of the horizontal stabilizer and elevator (whether asymmetric or not). Note that Q400 does not have a variable incidence tailplane:
The problem is that wing and tailplane stalls have opposite recovery procedures. Recovering from a tailplane stall entails full aft yoke, raising flaps and decreasing airspeed. Tough situation to find yourself in, particularly at low speeds.
Unfortunately, these recovery procedures are generally not part of airline training. Recognizing instantly that these wholly different recovery measures are required? Another whole new ball-game." It's like the 737 pilot's reaction to the rudder hardover in USAIR 427. If he'd eased off the rudder (and pedalled it L/R), that may have undone the jammed PCU's reversal. But instinctive sustained inputs always rule the day when instant inputs are required to resolve underway doomsday scenarios.
c. It's also noteworthy that the Q400 isn't difficult to load correctly but it does have a pretty narrow CofG range (despite what the Bombardier website says) and it's quite trim sensitive due to its length. Both spin and spiral instability are adversely affected by an aft CofG. An aft CofG will promote a flattish spin. It's worth noting that an aft CofG, even if within CofG limits, can become an adverse condition once tailplane/elevator authority has been partly compromised (and even excessive elevator trim travel caused?) by icing (on the tailplane and elsewhere).
So, to restate the case for an asymmetric icing cause for Control Loss: "Sudden unexpected autorotative roll during a turn, at well above the normal stall speed, is highly likely to generate an instinctive pilot reaction of opposite ("held-in") aileron. It's a well-known fact that use of aileron to re-instate wings level at the stall will more deeply embed the aircraft in the autorotative condition. This is why, even for wing drop in a wings level stall, the only solution (to prevent further wing-drop whilst pushing the yoke fwd to unstall the wings) is the secondary effect of rudder (i.e. roll) to prevent further wing-drop." That use of rudder to stop roll has never been instinctive in an unexpected sudden wing-drop/uncommanded roll scenario. In fact it got a bad rep for being the instigator of the A300 fin detachment in AA587 at Belle Harbor, Queens. If you're unaware that the wing-drop is the result of one wing stalling and you maintain a desperate wing-levelling aileron input (in an attempt to stop that roll), the result will be a flat spin. Use of increased engine power will also flatten the spin's pitch attitude.
Preferred Theory? If a tail stalls on any aircraft, the nose will drop, but the wings do not stall. The aircraft will continue to have considerable forward
movement. I suppose it
might theoretically even continue to nose over until inverted -
or, more likely, nose-drop ONLY UNTIL the higher IAS
restores tailplane effectiveness - i.e. unstalls it as the speed
increase lessens its AoA. Neither
outcome or development was evident in this accident.
The stick-shaker and stick-pusher
actuation suggests a simple wing-stall. It's hard enough to stop
a wingdrop in a clean, 1g, full back-stick induced stall; but
give each wing different aerodynamic characteristics and
superimpose the pilot's natural reaction (of immediate opposite
aileron) and you've got pro-spin controls (even though he went
nowhere near the full backstick that is characteristic of stall
training - so he's not thinking "stall/spin" nor
contemplating the use of rudder to prevent further roll).
There's been ample precedent amongst turboprops for this asymmetric icing scenario resulting in the stall speed between L&R wings differing significantly (as induced spanwise bysame-direction prop rotation). Such a large stall-speed differential between port and starboard wings is conducive to a very rapid snap-roll entry into an unrecoverable spin. Pilots, whether on autopilot or not will be totally caught out and will instinctively try to oppose the roll with aileron. That input will merely EMBED the dropping wing more deeply into a stalled condition. Application of power will just flatten the spin. Spins entered on autopilot will have the added penalty of having been trimmed (pitch and roll) deeply into the condition - and this will really complicate the already impossible task of attempted recovery.
Roselawn ATR / Aero Commander in NZ / ATR freighter in the Taiwan Straits /United Express, flight 2415 (BA-3101 turboprop)/ March 4, 1993, a Continental Express ATR-42/ EMB-120 Brasilia turboprop, (Comair 3272)/ a number of MU-2 accidents etc etc (see third cell down, in the table below).
Another aspect of investigating a roll upset icing accident should consider the possible effect of ice asymmetry and ice protection coverage. Propellers impart a swirling motion to the air that results in an asymmetric ice formation on the wing leading edges immediately downstream of the propellers. The downgoing blade side will cause more ice to accumulate farther aft on the upper surface and less further aft on the underside on that side. Conversely, the upgoing blade side will tend to cause more ice to accumulate farther aft on the lower surface and less on the upper surface.
Another aspect of investigating a roll upset icing accident should consider the possible effect of ice asymmetry and ice protection coverage. Propellers impart a swirling motion to the air that results in an asymmetric ice formation on the wing leading edges immediately downstream of the propellers. The downgoing blade side will cause more ice to accumulate farther aft on the upper surface and less further aft on the underside on that side. Conversely, the upgoing blade side will tend to cause more ice to accumulate farther aft on the lower surface and less on the upper surface. Apply this theory to each wing and then factor in the significance of the left and right props rotating in the same direction. The spanwise ice distribution will be lopsided to say the least. The Left and Right wings' upper and lower surfaces will end up with significantly different profiles and stalling speeds and characteristics. There will also be additional disymmetry in accretions on the port (versus starboard) fuselage and tail surfaces.
The computational fluid dynamics analysis derived graphic below visualizes this effect clearly on the horizontal surface. (from Newmerical Technologies International)
The blue area in the image below shows the area of the DHC-8- Q400 wing immersed in the propwash. It is approximately 43% of the wing area and represents a substantial percentage of the wing lift - so asymmetry due to ice formation in this area is worthy of careful consideration.
In certain icing conditions the helical flow field may result in not only asymmetry of ice formation, but may also form asymmetrically aft of the ice protection system coverage. When the engine power is reduced, the velocity of the propwash and the beneficial effects of the propeller flow will be reduced and the local angle of attack behind the propeller can increase suddenly. If the ice has caused a serious erosion of the normal safety margins, the result can be a lateral imbalance of lift and can contribute to a roll. In some cases the wing stall can spread across the wing and the ailerons changing the control characteristics of the airplane.
The picture below (from FAA FFFSCR report) was taken in an icing test behind an aerial tanker spraying water from a large “shower head” to determine the formation of ice. The water contains sea marker dye that appears yellow in the solid (ice) phase and fluorescent green in the liquid phase. Some green is visible on the viewer’s right at the very leading edge. Note the ice formation aft of the active area of the deicing boots on the viewers left (airplane’s right wing). This ice cannot be removed by the ice protection system. The DHC-8-Q400 propellers both rotate in the opposite direction so the effect would tend to be the mirror image of this.
If icing is found to be a causal factor, it's not known how the manufacturer addressed this issue on the accident airplane design and if, and/or how much, of a factor this may be.