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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 the Enhanced GPWS that uses a
terrain data base and compares current aircraft position to known terrain hazards, contained in
an onboard database. This system can provide a full 60 seconds of warning before terrain
impact.
C. CFIT Escape Maneuvers
A CFIT escape maneuver is usually performed in response to a GPWS warning and is a
procedure designed to remove an aircraft from a pending terrain contact in an expedient
manner. This maneuver is designed to remove the aircraft from harm by the use of maximum
or near maximum aircraft aerodynamic performance. Typically, the aircraft is in a descent.
Upon receiving a terrain warning, the pitch is increased to a value between 15 and 20 degrees
nose up until the stick shaker activates or maximum Angle-of-Attack (AOA) is reached. This
maneuver can be initiated anywhere from a clean cruise descent (280 to 300 KIAS), to a fully
configured condition at approach speed.
An industry task force has recommended the following general GPWS Terrain Warning (CFIT)
Escape maneuver.19
· React immediately to a GPWS warning
· Positively apply maximum thrust and rotate to the appropriate pitch attitude for your
airplane.
· Pull up with wings level to ensure maximum airplane performance.
· Always respect the stick shaker.
The task force report goes on to say:
Studies show that there is little difference in performance between a pull-up rate of 3
degrees/second and 4 degrees/second. Because of this, it is recommended that the standard
pull-up rate is 3 degrees/second. 20
1. AIRCRAFT WITH CONVENTIONAL FLIGHT CONTROLS
19 Industry task force. Appendix 4-D, section 4-D.1
20 Industry task force. Appendix 4-D, section 4-D.1.1
Revision 14.0 39
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 represents 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.
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 high 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, easily, and
repeatably achieve the maximum level of performance allowed by the envelope limiting
system. This ease of handling might, in certain cases, result in optimum CFIT escape
performance, even though full aerodynamic performance may not be achieved.
3. FBW AIRCRAFT WITH ÒSOFTÓ PROTECTION FEATURES
Boeing incorporates ÒsoftÓ limits in the design of their FBW flight control system. Soft limits
ÒsuggestÓ and warn when a limit is being approached by increased stick forces and by
introducing aural and visual warnings. With soft limits the pilot is warned, but then allowed to
stall, over-bank, over-stress or over-speed the aircraft, if necessary or desired.
In a CFIT escape maneuver with the 777, the Boeing recommended procedure (simplified) is
for the pilot to immediately select maximum thrust, rotate aggressively to 20 of pitch, and
retract the speed brakes.
In all cases, the pitch attitude that results in intermittent stick shaker or initial buffet is the
upper pitch attitude limit. When the flaps are not up or at slow speeds with the flaps up, the
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