The Conundrum Of American Airlines Flight 587

Air Safety Week,  Jan 31, 2005  

Tags: American Airlines

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NTSB wanted FAA, manufacturers and operators to do three things:

* Properly explain to pilots the design requirements for the vertical tail;

* Explain that a large rudder deflection in one direction followed by large deflections in the opposite direction can overload the tail; and

* Emphasize that on some aircraft types, relatively light pedal forces and small movements can easily output the maximum available rudder deflection.

Conversely, pilots must not be reluctant to use full rudder when appropriate, such as during an engine failure or gusty crosswind landings. Apparently NTSB's message has still not been well disseminated.

Tail Loads

Having now completed its investigation, NTSB confirms that the A300 was in fact flown in a manner that exceeded its design capabilities. Why were such loads never anticipated? Why were they not guarded against?

It is not practical to consider all eventualities during aircraft design. Instead, international design rules prescribe a few extreme conditions that are meant to encompass the worst loads that any aircraft will encounter during its operating life. These are called limit load conditions. For safety, limit loads are increased by a factor of 1.5 to give ultimate loads. Via analysis and testing, aircraft structures must be substantiated as strong enough to withstand those ultimate loads.

On the vertical tail, the case most akin to what happened on Flight 587 is known as the "Yaw Maneuver Condition." This involves assuming that rudder pedals are suddenly displaced to the maximum extent possible, limited by control stops or by a pilot effort of 300 lbs. Then, after the aircraft achieves an equilibrium sideslip angle with rudder fully deflected, the rudder control is suddenly returned to neutral.

Importantly, the condition is treated in isolation. That means the aircraft is not expected to sustain any other loads while the rudder is full over; neither from other controls nor from gusts. Nor, as happened on Flight 587 are repeated rudder inputs considered. It's an arbitrary condition dating back more than 50 years and until now it seemed to give conservative design loads for the vertical tail and rudder.

The Yaw Maneuver Condition dates back to an era when full rudder required great pilot effort; before the advent of power controls that now make that pilot effort limitation wholly irrelevant. Usually, power systems automatically restrict rudder deflection as speed increases and on some aircraft, including A300s, they also cut pedal movement.

For instance, NTSB observes that below 165 knots the A300-600 rudder has unrestricted deflection +/-30 degrees requiring a pilot force of about 65 lbs. to move the rudder pedals about 4 inches (100mm). However, at the accident airspeed of 250 knots, a limiter restricts rudder travel to about +/-9.3 degrees and a force of only 32 lbs. pushes rudder pedals through their reduced range of about 1.2 inches. This sensitivity makes it very easy for a pilot to command full and/or sequential rudder movement, especially as leg muscles are physiologically insensitive to force and position.