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engines.
Some designs have limited pilot authority by limiting allowable thrust to less than available
thrust. At least one operator of the Airbus A319 (not the manufacturer) has elected to limit
the thrust allowable to the pilot to 22,000 pounds per engine. The aircraft is certified to
23,500 pounds thrust per engine, an additional 6.8% per engine. The engines are actually
capable of producing 30,000 pounds of thrust each. That means that there is available in the
engine, but not to the pilot, an extra 20% of engine thrust. Full rated engine thrust however,
may not be desirable. In the case of the A319, high thrust settings under certain conditions
can generate problems with Vmc, adverse pitching moments and elevator authority. However,
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for most designs, an aircraft can accommodate a 30% increase in thrust (over the base design),
without adverse effects.3
Reduced thrust settings provided by FADEC systems are appropriate for routine line
operations. However, the inability of the pilot to access full certified thrust when needed to
recover from a wind shear encounter, an event where full performance is needed, or other
emergency events requiring additional thrust may spell disaster for all aircraft occupants. If
there were to be an accident where it could be shown that the extra 6.8% of thrust would have
made a difference, the tort lawyers would have a strong case for a negligent design or operator
decision. The current operational reasons to limit the thrust available to the pilot are to reduce
fuel burn and increase engine life. FADEC systems can be programmed to force the pilot to
make a reduced thrust takeoff on every event, despite conditions that may warrant otherwise.
This decision was not made with the concurrence of the pilot in command. The pilot is not
allowed to select maximum certified thrust (the thrust level that the manufacturer thought was
appropriate for the aircraft at that weight) to achieve a higher level of safety for a takeoff with
clutter, to have greater obstacle clearance capability or better performance in case of wind
shear.
The Boeing 777-200 also limits pilot authority in a similar manner. One of the three available
engines, the Pratt & Whitney 4000, is rated to produce between 74,500 and 84,600 pounds of
thrust. The customer specifies the desired thrust level (74,000, 77,000, or 84,000), and this is
then pin selected on the FADEC and marked on the engine data plate. On heavy weight
versions of the 777, the thrust is set at the higher level. The lower thrust is used for the lighter
weight versions, but the crew is not allowed to access the higher thrust capability of the
engine, even in an emergency.
But not all designers have agreed with this limitation to pilot authority. For example, the
McDonnell Douglas designed MD-90 allows the pilot to select an emergency thrust level by
pushing the throttles through a brake bar. Many MD-90s are equipped with IAE V2500
engines rated at either 25,000 or 28,000 pounds of thrust per engine (customer option).
However, the V2500 engine is capable of over 30,000 pounds of thrust. By pushing through
the gate on the MD-90 throttle quadrant, the pilot is able to select full rated N1 thrust of over
30,000 pounds with full engine overspeed protection (both N1 and N2). This extra thrust is
provided to the pilot on the occasion that the pilot determines it is needed to counter wind
shear or otherwise escape ground contact.
Philosophically, the design team at McDonnell Douglas felt that they could not justify a
scenario which saw the aircraft impacting the terrain with both engines only producing 25,000
pounds of thrust, when the engines were capable of over 6,000 more pounds of thrust, with
full overspeed protection (Note: there may be other factors to limit the max thrust such as
Vmc considerations.).
3 ALPA-Airbus meeting April 1998.
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Yet another case for full pilot authority is found in the MD-90 thrust reverser development.
The MD-90 initially limited the amount of reverse thrust available to the pilot based upon
airspeed. This was done to reduce engine damage at low airspeeds due to FOD and also for a
rudder power consideration single engine. Unfortunately, it also prevented the pilot from
utilizing an emergency source of stopping power, such as might be needed on an icy ramp.
Fortunately, the manufacturer changed this design (suggested by ALPA) to provide the pilot
with full reverse power, if required. Full emergency reverse (full rated N1) is provided if the
pilot pulls the reverse lever through the aft reverse detent.
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5.1 Effects Of Additional Thrust On Climb Performance
Additional thrust can have a significant impact on climb rates. The basic equation to calculate
　
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