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CFIT maneuver performance capabilities of aircraft with hard versus soft Fly-By-Wire (FBW)
flight control systems. To obtain this data, simulated CFIT avoidance maneuvers utilizing a
Boeing 777-300 and an Airbus A330-200 were performed. These tests were performed at the
Boeing Flight Test Facility in Seattle, Washington and the Airbus Flight Test Facility at
Toulouse, France.
This flight test had a two-fold purpose. The first was to evaluate the effectiveness and
appropriateness of a recovery technique that was developed for convention aircraft without
regard for the flight envelope protections incorporated in modern FBW aircraft. The second
purpose was to develop and/or evaluate CFIT escape maneuvers that utilize the maximum
capability of the aircraft afforded by the protections incorporated in their respective FBW
flight control systems.
6.3.2 CFIT Maneuver
A CFIT escape maneuver is a procedure designed to remove an aircraft from a pending terrain
contact as judiciously as possible. This maneuver is designed to protect the aircraft, while
demanding maximum performance. Typically, the aircraft is in a descent, and upon receiving a
terrain warning, the pitch is increased to a value between 15 to 20 degrees nose up, until the
stick shaker or maximum AOA (angle of attack) is reached. This maneuver can be initiated
anywhere from a clean cruise descent (280 to 300 KIAS), or fully configured at approach
speed.
Transport aircraft are equipped with a variety of Ground Proximity Warning Systems
(GPWS). These systems will usually, depending upon sophistication, warn the flight crew 5
to 60 seconds before impact. The most advanced system is Enhanced GPWS which uses a
terrain data base and compares current aircraft position to known terrain hazards stored in an
onboard data base. This system can provide a full 60 seconds of warning before terrain
impact. However, at the point where a GPWS warning is issued to the flight crew, aircraft
performance can become the critical factor. The pilot must have the authority necessary to
achieve the maximum obtainable aircraft performance.
6.4 Aircraft Flight Control Design
6.4.1 Aircraft With Conventional Flight Controls
For an aircraft with conventional flight controls only mechanical stops and control forces limit
pilot authority. In reality, flight controls have set mechanical limits; that is, the pilot cannot
demand an elevator deflection of 90 degrees, and flight control power has definite physical
limits. Flight envelope excursions are possible and are typically indicated by the onset of high
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stick forces or stick shaker activation. The typical CFIT escape maneuver for an aircraft with
conventional flight controls, requires the pilot to select TOGA thrust, rotate at a smooth rate
of 3 degrees per second (to avoid overstressing or stalling the aircraft) to a pitch attitude of
between 15 to 20 degrees nose up. This pitch attitude is maintained until the stick shaker
activates or terrain clearance is assured. The stick shaker gives the pilot an indirect indication
of optimum AOA and must be respected to effect recovery. Due to speed and thrust
changes, with the resultant variable and usually out of trim stick forces; flying the stick shaker
activation angle to maintain optimum AOA, can be quite difficult.
The Cali accident report states, Òif the pitch attitude had been varied to perfectly maintain the
stick shaker activation angle (optimum AOA), the airplane could have been climbing through a
position that was 300 feet above the initial impact point.11Ó Cali is an example of a CFIT
accident where a FBW design may have made a difference.
6.4.2 FBW Aircraft With ÒHardÓ Protection Features
Airbus incorporates ÒhardÓ limits in the design of their FBW flight control system. Hard
limits prevent the pilot from exceeding the designed flight envelope of the aircraft. That is, the
aircraft may not be stalled, over-banked, over-stressed, or over-sped. In other words, the
designed aircraft envelope is maintained and protected.
The Airbus design allows the pilot to obtain, in a repeatable fashion, a consistent level of
aircraft performance. However, the pilot may be prevented from obtaining maximum aircraft
aerodynamic performance.
The procedure for the CFIT escape maneuver in the Airbus aircraft as recommended by
Airbus, is for the pilot to pull full back on the stick and apply TOGA thrust. Speed brakes if
extended, will automatically retract.
Control laws either stabilize the AOA at an optimum value or adjust pitch rate to obtain
maximum allowed g. With the Airbus CFIT escape maneuver pilots can quickly and easily
achieve a repeatable consistent level of performance allowed by the envelope limiting system.
　
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