A Tale of Two T-Tails

Air Safety Week, Sept 18, 2006

Inescapably Floored by Design Flaws

When Russia's Pravda disclosed that the Pulkovo TU154 (reg: RA-85185) that crashed on Aug. 22, forty minutes after takeoff, had been trying to outclimb a thunderstorm, the air safety cognoscenti were immediately aware of a likely scenario for what had transpired. The upshot: the aircraft was doomed by the kinds of design flaws that call for eternal vigilance from air safety regulators.

The latest data from the MAK (Interstate Aviation Committee) reports that Flight 612 had step-climbed to 12,400 m (41,000ft). Maximum allowed weight at 12,100 m is 85t for the TU154. Take-off weight must have been around 93.5t, given the distance from Anapa to St. Petersburg and with 160 passengers. This would mean a weight of about 88t at the time of the crash.

The thunderstorm was reported as having been a heavy one, reaching up to 12-15 km. It looked like the aircraft had stalled and entered a flat spin when it encountered turbulence at a low indicated airspeed (IAS), because it was way too high up for its weight. This was confirmed two weeks later in a statement by the Ukrainian Attorney General.

Flat Spins

There have been three previous fatal flat spin accidents in large jet airliners with passengers. All of them were TU154s. There's a transcript of the last one, and it's chilling. After the stall warning, and even though the crew reacted immediately, it was already too late. Apparently the Soviet authorities didn't believe in stickpushers, or even the earlier lesser measure of alerting stickshakers (apparently the TU134 lacks a stickshaker as well).

Any T-tail aircraft has a critical wing Angle of Attack (AoA) where a horizontal stabilizer mounted atop the vertical fin, and the all-important rudder also, will be blanked by the turbulent airflow off the wings. Experience shows that the resulting embedded stall outcome will produce an attitude that you won't recover from -- not without a tail-chute deployment that could change the balance of forces. The critical wing AoA for deep stall on the Trident, 727 and DC9 was quite high, around 30 to 35 degrees. The major player in determining this is the wing-to-T-tail relationship.

However, this crisis AoA can also be affected by icing buildups, engine thrust, IAS, leading and trailing edge flap, speedbrake, stabilizer position, Centre of Gravity placement, as well as any dynamics in the pitching plane -- e.g., back-stick input at entry to the condition or the rapid onset of induced drag as an autopilot's auto-trim seeks to maintain a selected altitude). As a clincher it's known that, courtesy of its tail fuel tank, the TU154 is usually flown fairly tail-heavy in the cruise. The MD-11 operates similarly as a way of minimizing aerodynamic drag. Entering a stall/spin in that aft weight-trim would be like a tsunami washing over an earthquake zone.

Deep Stalls and Bogus Stick-Pushes

A Deep Stall occurs when the turbulent wake of a stalled mainplane "blanks" the horizontal stabilizer, rendering the elevators ineffective and preventing the aircraft from recovering. For T-tails, the deep stall is the equivalent of the helicopter's vortex ring condition, as they can both generate very high rates of descent in a near normal wings-level flight attitude. It's only the relative airflow that counts. For a T-tail aircraft, the precursor to a flat spin is usually a deep stall. The British BAC One-Eleven prototype was lost on an early test flight in 1963 due to a deep stall, well before the two later Trident deep stalls.

The first crash of a 727 (United, near Chicago, August 1965) has never been properly explained and may have been a deep stall on leveling off as well. However, without any thrust, drag or lift asymmetry, there need not be any follow-on autorotation (i.e., a spin entry). The BEA Flt548 Trident deep stall at Staines UK (G-ARPI on June 18, 1972) occurred two minutes after takeoff when the leading-edge lift augmentation droop (aka slat) was prematurely retracted by the 22-year-old second officer.

Despite the stick-shaker operating its warning, this led to a series of stalls and eventually a deep stall from which recovery was impossible. The Trident stick-pusher was prone to false alarms so its actuation together with the stick-shaker did not necessarily alert pilots to a bona fide cause, such as early droop retraction. On G-ARPI, the pusher's intervention was instinctively cancelled as a nuisance warning well before it could be effective. Cardiologists found that the captain had probably had a simultaneous heart attack. Subtle pilot incapacitation wasn't in the 1960s first officer syllabus.

Stalling for (All) Time

In an earlier 1966 Trident crash, the aircraft was carrying out the first of a series of production test flights to qualify for a Series Certificate of Airworthiness. After completing a large part of the required tests, the stall tests were begun. Three approaches to stall were made to check the stall warning and stall recovery systems. The fourth stall test was made at an altitude of 11,600 ft in the landing configuration and with the stall warning and recovery systems inoperative. The Trident entered a deep stall with the nose going up to a 30[degrees] to 40[degrees] attitude. The aircraft yawed to the left, the right wing dropped, and the plane went into a flat spin to the right.