Reverse Thrust

Following an engine failure, the CSU will decrease the blade angle to fine/flat pitch in an attempt to maintain RPM as the propeller’s rotational velocity decreases. If the prop were the reversible pitch type, the blades would go into reverse pitch if it were not for the fine/flat pitch stop or the autofeathering system, if installed. A squat switch on the undercarriage, or release triggers on the throttle, or some other method is used to remove the fine/flat pitch stop in order to allow the prop blades to move into reverse pitch. Needless to say, reverse pitch should never be selected until the aircraft is firmly on the ground, due to the possibility of an excessively high and dangerous rate of decent. If one prop fails to return to forward thrust, a severe asymmetric condition will result. This has actually happened with fatal results.

Full throttle produces full thrust but zero thrust is produced with the throttle slightly open. When the throttle is fully closed, a small amount of negative thrust or prop drag is present. With the aircraft firmly on the ground, reverse thrust is applied by retarding the throttle through the gate or detent to idle reverse. This action will turn the blades via the CSU to a fixed negative blade angle of around 30 degrees past the fine/flat pitch stop. At idle reverse, the prop will produce about 60% of the maximum reverse thrust available. Further retardation of the throttle lever, will power the prop to a higher RPM than reverse idle to produce the total amount of reverse thrust available. Full aft movement of the throttle lever produces full reveres thrust caused by the increase of engine power absorbed by the prop. Reverse idle may provide sufficient braking force on long runways without the need to go to full reverse thrust. A turboprop produces about one third of its maximum shaft horsepower (SHP) when full reverse is applied, reducing the landing roll by about one quarter to one third of the unassisted reverse thrust landing distance.

Another advantage of reverse thrust is the fact it destroys the wing’s lift placing more weight on the undercarriage wheels for increased braking. The disadvantage here is the degraded elevator effectiveness; all the wheels should be firmly on the ground to prevent the nose-wheel dropping on quite.

To achieve maximum benefit of reverse thrust it should be applied fully and early as possible in the landing roll. It is more effective at higher speeds just after touchdown than it is when the landing roll is nearly complete. This is due to the fact, the aircraft’s kinetic energy is destroyed quicker and the reverse thrust force is greater due to the addition of the aircraft’s forward velocity. As the aircraft decelerates, the prop blade’s negative angle of attack reduces with the result, the reverse thrust also reduces. Reverse thrust should be cancelled when the aircraft’s ground speed is down to around 40-50 knots. However, there are some exceptions to this rule; check the aircraft’s flight manual (POH). Reverse thrust should be avoided if possible when operating on unsealed airstrips due to gravel damage to the blade’s leading edge and foreign object damage to the engine.

It is possible to taxi the aircraft backwards using reverse thrust, but the brakes should be used with caution to prevent the aircraft tipping on its tail. Visibility behind the aircraft is also a problem

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