Gas Turbines - Reverse Thrust — Page 329, Lesson 420

Let's start with the five safeguards built into the selection of Reverse Thrust. These are the built-in protections that stop you from doing something dangerous with the reverser system. First, reverse thrust cannot be selected until the throttle lever is at idle. That means the engine must be at its lowest power setting before you can even arm the reverser. Second, reverse thrust cannot be activated until the aircraft has weight on the mainwheels. This is the air/ground logic interlock. The system checks that the aircraft is actually on the ground before it will let you use reverse thrust. Third, rpm in Reverse cannot be increased above idle until the reverse thrust doors are in the deployed, or Reverse Thrust, position. So even after you select reverse, you can't spool the engine up until the doors have physically moved into place. Fourth, if while Forward Thrust is selected the reverser doors inadvertently move to the deployed position, the throttle may automatically close to idle. This is a protection against an uncommanded deployment — if the doors start moving when they shouldn't, the system cuts the power. Fifth, if while Reverse Thrust is selected the reverser doors inadvertently move to the stowed, or Forward Thrust, position, the reverse thrust lever will automatically go to the reverser deploy position. So the lever tries to command the doors back to where they should be. Now, restrictions of use. While there is normally no restriction on the upper speed at which reverse thrust can be selected, some aircraft have systems fitted which place a restricted minimum speed of operation on the reverse thrust system. Earlier I described how the lower cascade vanes of the clamshell door system were angled forwards and outwards. That was to minimize the chances of debris and hot gases being re-ingested into the engine. But there is a clear danger. Despite the angle of the cascade vanes, if the aircraft is only moving forwards slowly, or is stationary, the depression in the engine air intake will overcome the deflection applied to the exhaust gas stream — and any associated debris — and suck it into the compressor. That could have catastrophic consequences for the engine. To prevent this, the Standard Operating Procedure, or SOP, on some aircraft is to reduce the reverse thrust lever to the reverse idle position at typically 60 to 80 knots. Then, at a speed where there is no further benefit from maintaining that Idle Reverse position — usually about 50 knots — the reverse thrust lever is returned to the stowed position. Now, ground manoeuvring reverse thrust is not normally used. When it is in use, engine indications must be closely monitored, in particular for excessive EGT — that's exhaust gas temperature. Care must be exercised when increasing reverse rpm that the engines respond symmetrically, because adverse yaw can be induced. There may also be a performance limitation imposed if one engine thrust reverse system is inoperative. The total reverse capability will be reduced, and on a two wing pylon mounted engined aircraft, it may mean the good reverser may not be operated either, because of the asymmetric effect. So to tie it together: the five safeguards protect against uncommanded deployment and premature use, and the speed restrictions protect the engine from re-ingesting its own exhaust and debris.