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This ease of handling might, in certain cases, result in optimum CFIT escape performance,
even though full aerodynamic performance may not be achieved.
The argument can be made that pilot authority is limited in the ÒhardÓ design by the fact the
pilot is prevented from exceeding the limits of the flight envelope. The Airbus design allows
the pilot to rapidly obtain maximum allowed aircraft performance to avoid ground contact.
However, the pilot is prevented from obtaining all possible aircraft aerodynamic performance.
That last bit of available but not attainable performance may be all that is necessary to avoid
ground contact.
11 Controlled Flight into Terrain, American Airlines flight 965. Aeronautica Civil of the Republic of Columbia.
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6.4.3 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 current Boeing recommended procedure is to
aggressively apply maximum thrust Roll wings level and rotate to an initial pitch attitude of
20 degrees. Retract speed brakes if extended. 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 pitch limit indicator (PLI) provides a visual
reference of the pitch attitude limit. Follow flight director TO/ GA guidance if available.12
The addition of the PLI in the Boeing 777 FBW design allows the pilot to easily and
consistently achieve the optimal aircraft attitude. On the 777 the pilot directly controls pitch
attitude and pitch rate. High pitch rates can be attained by the pilot to quickly and precisely
place the aircraft at optimum AOA. Although easier than for conventional aircraft, accurately
maintaining the PLI still requires a reasonable degree of pilot technique. If ground contact is
imminent the pilot can obtain the full aerodynamic performance of the aircraft. High stick
forces are required to pull the aircraft into a stall; but the pilot receives numerous warnings and
indications of the pending stall condition. Other than a ramp up of stick force there is no
indication that the aircraftÕs g limit has been reached or exceeded. The authority to obtain
maximum g is only limited by the feel system and control power. With this design the pilot is
allowed to obtain the maximum aerodynamic capability of the aircraft. Although not explicitly
stated in the procedure, BoeingÕs intent is for the pilot to aggressively (greater than 3
degrees/second) rotate to the initial 20 degree pitch attitude.13
It is the committeeÕs opinion that the CFIT recovery capability on the 777 could be enhanced
if the aircraftÕs Primary Flight Computers (PFC) were designed to recognize aggressive pilot
inputs as a desire for maximum aircraft performance. The PFCs would then provide maximum
pitch rate consistent with AOA or g limits (depending on airspeed). If the resultant aircraft
performance is not sufficient, the pilot could then pull to the full aerodynamic capability of
the aircraft. Additionally, automatic speed brake retraction, in the event of a go around or
CFIT maneuver, should be provided in the 777 design. This system although somewhat
complex mechanically, can be implemented since the PFCs will control any undesired pitch
excursions.
6.4.4 CFIT Recovery Conclusions/Recommendations
12 Boeing 777 Flight Crew Training Manual, 1.26June 01, 1997.
13 Personal conversation John Cashman Feb 18, 1999.
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From the data gathered in the evaluation, there was not a distinct advantage of the B777 soft
limits vs the A320/330 hard limits for CFIT recovery for open loop performance. However,
closed loop evaluations showed that the pilots could achieve more consistent performance
results as well as achieve target pull out parameters more quickly in the A320/330 than the
B777. Even with the B777 soft limit features, pilots were able to use abrupt pitch inputs
without fear of overstress or stall. Both aircraft types offered better handling during CFIT
recoveries than conventional aircraft since the FBW design features allowed the pilot more
precise control of pitch rate and g onset rate than with conventional flight controls.
6.4.5 Conclusions
· The A330 full aft stick CFIT recovery vs 3 deg/sec pull gave better and more
　
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