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drag with the nosewheel on the ground and the engines
continue to produce forward thrust with the power
levers at idle. While wheel brakes normally can cope,
there is an obvious need for another speed retarding
method. This need is satisfied by the drag provided by
reverse thrust.
A thrust reverser is a device fitted in the engine
exhaust system which effectively reverses the flow of
the exhaust gases. The flow does not reverse through
180°; however, the final path of the exhaust gases is
about 45° from straight ahead. This, together with the
losses in the reverse flow paths, results in a net efficiency
of about 50 percent. It will produce even less if
the engine r.p.m. is less than maximum in reverse.
Normally, a jet engine will have one of two types of
thrust reversers, either a target reverser or a cascade
reverser. [Figure 15-19] Target reversers are simple
clamshell doors that swivel from the stowed position at
the engine tailpipe to block all of the outflow and
redirect some component of the thrust forward.
Cascade reversers are more complex. They are
normally found on turbofan engines and are often
designed to reverse only the fan air portion. Blocking
doors in the shroud obstructs forward fan thrust and
redirects it through cascade vanes for some reverse
component. Cascades are generally less effective than
target reversers, particularly those that reverse only
fan air, because they do not affect the engine core,
which will continue to produce forward thrust.
On most installations, reverse thrust is obtained with
the thrust lever at idle, by pulling up the reverse lever
to a detent. Doing so positions the reversing
mechanisms for operation but leaves the engine at idle
r.p.m. Further upward and backward movement of the
reverse lever increases engine power. Reverse is
cancelled by closing the reverse lever to the idle
reverse position, then dropping it fully back to the
forward idle position. This last movement operates the
reverser back to the forward thrust position.
Reverse thrust is much more effective at high airplane
speed than at low airplane speeds, for two reasons:
first, the net amount of reverse thrust increases with
speed; second, the power produced is higher at higher
speeds because of the increased rate of doing work. In
other words, the kinetic energy of the airplane is being
destroyed at a higher rate at the higher speeds. To get
maximum efficiency from reverse thrust, therefore, it
should be used as soon as is prudent after touchdown.
When considering the proper time to apply reverse
thrust after touchdown, the pilot should remember that
TARGET OR CLAMSHELL REVERSER
CASCADE REVERSER
Figure 15-19. Thrust reversers.
Ch 15.qxd 5/7/04 10:22 AM Page 15-14
15-15
some airplanes tend to pitch noseup when reverse is
selected on landing and this effect, particularly when
combined with the noseup pitch effect from the
spoilers, can cause the airplane to leave the ground
again momentarily. On these types, the airplane must
be firmly on the ground with the nosewheel down,
before reverse is selected. Other types of airplanes
have no change in pitch, and reverse idle may be
selected after the main gear is down and before the
nosewheel is down. Specific procedures for reverse
thrust operation for a particular airplane/engine
combination are contained in the FAA-approved
Airplane Flight Manual for that airplane.
There is a significant difference between reverse pitch
on a propeller and reverse thrust on a jet. Idle reverse
on a propeller produces about 60 percent of the reverse
thrust available at full power reverse and is therefore
very effective at this setting when full reverse is not
needed. On a jet engine, however, selecting idle
reverse produces very little actual reverse thrust. In a
jet airplane, the pilot must not only select reverse as
soon as reasonable, but then must open up to full power
reverse as soon as possible. Within Airplane Flight
Manual limitations, full power reverse should be held
until the pilot is certain the landing roll will be
contained within the distance available.
Inadvertent deployment of thrust reversers is a very
serious emergency situation. Therefore, thrust reverser
systems are designed with this prospect in mind. The
systems normally contain several lock systems: one to
keep reversers from operating in the air, another to
prevent operation with the thrust levers out of the idle
detent, and/or an “auto-stow” circuit to command
reverser stowage any time unwanted motion is
　
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