Jammed Elevators the Result of a ‘Grandfathered’ Design

Wed, Feb 16, 2011 — David Evans

Articles, Featured

Straining hard to keep the control yoke forward, the captain and first officer were able to overcome the jammed elevators on their Tailwind Airlines B737-400 after an uncommanded pitch-up to 40º. The airplane was just 20 feet off the ground, approaching to land at Turkey’s Diyarbir airport.

The flight crew performed a go-around and managed to land the airplane with no injuries to the 159 passengers and flight attendants. The two pilots had to push constantly on the control yoke, as the elevators were jammed by a piece of metal about 0.2 x 0.14 inches in one of the elevators’ two power control units (PCUs, one on each side).

The elevator PCUs are located in the tail of the airplane

The U.S. National Transportation Safety Board (NTSB) assisted Turkish authorities in investigating this incident involving Turkish-registered Tailwind Airlines. On 10 February 2011 the NTSB issued to the Federal Aviation Administration (FAA) its recommendations, which certainly suggest that the venerable B737-400 is lacking in safety features found on many other model airliners.

The B737-200 through -500 series does not feature a manual override should the elevator become jammed, despite the fact that either a rate jam or a position jam should be considered a catastrophic hazard.

The NTSB examined service difficulty reports (SDR) and found four accounts of B737-300 through -500 elevator jams. As the NTSB said:

“Although none of these events resulted in an accident, they highlight the fact that binding or jams of the elevator system do occur in flight, can result from numerous causes (including improper maintenance performed on the airplane), and can present the flight crew with controllability hazards.”

The NTSB found that many airplanes feature a manual override so pilots will maintain control when a system malfunction occurs. To cite just some of the airliner models with override mechanisms in the elevator system: Boeing B717, B747, B757, B767 and B777 airplanes; Embraer 120, 145, 170 and 190 models; Bombardier CRJ-200, CL-600, DHC-8 and Q400 airplanes; and ATR-42 and -72 airplanes. Moreover, the “next generation” B737-600 through -900 series incorporates several override mechanisms.

But the “classic” B737-300 through -500 were not retrofitted with override mechanisms, and when one PCU is jammed by pieces of metal or other foreign object debris (FOD), the other PCU is unable to exert sufficient force to overcome the condition.

Before being added to Tailwind’s fleet, the aircraft went through scheduled maintenance at AAR Services in Oklahoma. While undergoing the “C” check, the left side elevator failed the free play check. A number of parts were replaced; the free play check was passed. But, obviously, all debris was not cleaned up, and six months later, on approach to land at Diyarbir, the left elevator jammed.

Tailwind Airlines B737-400

The airplane did not incorporate a manual override feature. The B737-300 through -500 series was a derivative of the B737-100/200 model. This airplane was certificated by the FAA in 1967. At the time, the regulations did not require consideration of a single point failure (such as a single PCU jam) as long as the failure mode was considered extremely remote (e.g., probability of occurrence one in a billion flight hours). The B737-300/500 was certificated as a derivative of the B737-100/200. As such, it was grandfathered to the original certification standard. As shown by the recent SDRs, the airplane’s elevator control system falls well short of the one in a billion standard.

Federal Aviation Regulations (FARs) modified since 1967 require an elevator jam to be handled by the crew “without requiring exceptional piloting skill or strength.” This requirement is the genesis of the manual override feature on airplanes certificated since.

The whole process of grandfathering derivative designs to an earlier, less demanding, certification requirement bears review and change. Here’s an airplane with potentially a “catastrophic hazard” in daily revenue service with unsuspecting pilots.

There is another wrinkle. The PCUs for actuating the wing ailerons are of the same design – with a key difference. The aileron PCUs are located in the main landing gear wheel well. When the landing gear is extended, the aileron PCUs are vulnerable to damage from environmental debris or tire failure. Accordingly, protective soft covers were installed over both aileron PCUs.

These protective covers would also protect the elevator PCUs from FOD.

The NTSB recommended the installation of these protective covers to the aileron PCUs of B737-300 through -500 models, and a manual override such as that found on other models certificated since.

One could argue that the NTSB recommendations, although good, don’t go far enough. The whole process of the FAA approving designs, and not eliminating grandfathering to earlier models of the same airplane, needs to be changed. And if protective covers are used on one application, why aren’t they employed on all aircraft, for all types of actuators? For example, on U.S. Air Force C-141 and C-5 cargo jets, the jackscrew for the horizontal stabilizer is protected by a bellows-type device that blocks FOD.

And if the NTSB didn’t issue the recommendations, would the B737-300 and -500 have continued with an uncorrected latent catastrophe? The answer: a highly probable “yes”. So much for the FAA’s vaunted ethic “one level of safety.” Lessons learned are simply not applied to every aircraft of the same basic design.

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