Aircraft Pilot Coupling Needs to be Avoided During Design

Air Safety Week, Nov 1, 2004

Test techniques have been developed to assess an airplane's susceptibility to aircraft pilot coupling (APC).

The test methods are contained in an advisory circular (AC) published March 31, 1998, by the Federal Aviation Administration (FAA). This document, AC 25-7A, titled "Flight Test Guide for Certification of Transport Category Airplanes," was produced years after the Airbus A300-600 was certified for service in 1988. At the time of its certification, an earlier version of this AC, No. 25-7, was in effect. This earlier document discussed the need to minimize susceptibility to APC and the A300-600 met all certification standards in force at the time.

The 1998 edition of the AC raises the bar for certification purposes by laying out more specific criteria for the evaluation of an airplane design's susceptibility to APC. It features both a numerical rating scale and a characterization ranging from "satisfactory" to "unsatisfactory."

By its nature, APC can be a pilot-entrapment latent flaw, the disclosure or discovery of which is the purpose of flight tests.

The rating scale is to be used by test pilots. To cite from the AC:

* "Service experience has shown that compliance with only the quantitative, open-loop (pilot-out-of-the-loop) requirements does not guarantee that the required levels of flying qualities are achieved. "

* "Therefore, in order to ensure that the airplane has achieved the flying qualities required by [Federal Aviation Regulations]

US dollars 25.143(a) and (b), the airplane must be evaluated by test pilots conducting high-gain, closed-loop tasks to determine that the potential of encountering adverse APC tendencies is minimal."

* "The evaluation of flying qualities discussed herein ... should be accomplished by at least three test pilots."

* "Only the pilot's rating of the APC characteristics is needed."

Based on the known sequence of events in the 2001 loss of American Airlines Flight 587, the pilot introduced abrupt control maneuvers.

If these movements triggered an APC event, it was necessary for the pilot to release or freeze the flight controls. The four rudder reversals and dynamic, opposing movements on the control wheel, show that he did the opposite - a symptom of a pilot's unfamiliar response to APC (rather than what the Flight 587 accident pilot appears to have thought - that the airplane was encountering an unusually strong wake vortex).

Throughout the Flight 587 investigation, Airbus officials have consistently declared that the accident could have been avoided had the pilot stopped inputting rudder reversals and allowed the airplane to fly through unexceptional wake turbulence.

As Airbus asserted in its submission to the National Transportation Safety Board (NTSB): "The accident would not have happened had the first officer simply taken his feet off the rudder pedals at any time prior to ... structural overload." (See ASW, March 29) Short of releasing or freezing the controls, pilots can endeavor to reduce or escape oscillations by reducing the gain. Regarding the A300-600's sensitive rudder system, modulating - or reducing - the control inputs may be difficult. During last week's final hearing on the Flight 587 accident, the NTSB's Malcolm Brenner said, "The system is so sensitive, it would be difficult for a pilot not to hit the stops."

AIRCRAFT PILOT COUPLING (APC) RATING SCALE

Descriptive Rating: Satisfactory

Numeric Rating Scale : 1

APC Characteristics Description: No tendency for pilot to induce undesirable motion.

Descriptive Rating: Satisfactory

Numeric Rating Scale : 2

APC Characteristics Description: Undesirable motions (overshoots) tend to occur when pilot initiates abrupt maneuvers or attempts tight control. Those motions can be prevented or eliminated by pilot technique. (No more than minimal pilot compensation required)

Descriptive Rating: Adequate

Numeric Rating Scale : 3

APC Characteristics Description: Undesirable motions (unpredictability or over control) easily induced when pilot initiates abrupt maneuvers or attempts tight control. These motions can be prevented or eliminated but only at sacrifice to task performance or through considerable pilot attention and effort. (No more than extensive pilot compensation required)

Descriptive Rating: Controllable

Numeric Rating Scale : 4

APC Characteristics Description: Oscillations tend to develop when pilot initiates abrupt maneuvers or attempts tight control. Adequate performance is not attainable and pilot must reduce gain to recover. (Pilot can recover by merely reducing gain)

Descriptive Rating: Unsatisfactory

Numeric Rating Scale : 5

APC Characteristics Description: Divergent oscillations tend to develop when pilot initiates abrupt maneuvers or attempts tight control. Pilot must open loop by releasing or freezing the controller.

Descriptive Rating: Unsatisfactory

Numeric Rating Scale : 6

APC Characteristics Description: Disturbance or normal pilot control may cause divergent oscillation. Pilot must open control loop by releasing or freezing the controller.