B737 Reverse Thrust use during aborts and landings
The B737-Classic and NG FCTM states: "Maintain reverse thrust as required until the airspeed approaches 60 knots. At this point start reducing the reverse thrust so that the reverse thrust levers are moving down at a rate commensurate with the deceleration rate of the airplane. The thrust levers should be positioned to reverse idle by taxi speed, then full down after the engines have decelerated to idle."
In a similar statement the B737 Instructor Pilot Guide published in August 1976, states: "At 60 knots reduce thrust to be at idle reverse at taxi speed"
From personal observations both in the aircraft and simulator, pilots rarely wait until the N1 is at idle reverse before positioning the reverse thrust levers full down. In fact many rely on the PNF to call "60 knots" before taking action to reduce reverse and frequently we see the PF hanging on to full reverse if the PNF for some reason forgets the support call of "60 knots" In other words the PNF is relying on the 60 knot call before taking action. The 60 knot call is a support call - not an action call.
While Boeing state an airspeed indication of 60 knots, I wonder if landing into a strong headwind component it is more appropriate to consider the use of 60 knot ground speed - rather than 60 knot airspeed.
Obviously ingestion and engine instability is the concern; which is why full reverse is not held to low speeds below 60 knots unless it is a real emergency. Consider a rejected take off where it is often the case the RTO position of the auto-brakes is so effective that the ground-speed reduces rapidly, so by the time reverse thrust has spooled up to full reverse the aircraft is below 80 knots where reverse is not that effective.
In fact the difference between max rejected take off using RTO and no reverse and RTO with reverse, is only 170 feet on a dry runway if all actions are carried out promptly. Different of course on wet or slippery runway.
If taking off into a 30 knot headwind and a RTO is conducted as per book, then at the call of 60 knots by the PNF, the ground speed would be 30 knots - thus significantly increasing the chances of ingestion since full reverse is still being used at 30 knots ground speed.
Boeing does not address this point, so maybe it is statistically irrelevant.
But in terms of good airmanship, it is possible that high reverse thrust in use at speeds approaching taxi ground speed of (say) 20 knots, could be detrimental to the engines.
After all no one would consider using high reverse thrust when taxiing - for all the obvious reasons, yet we accept full reverse at 30 knots ground speed during landing if landing into a 30 knot headwind (60 knot IAS v 30 knot ground speed)
Why would the manufacturer Boeing therefore use 60 knots indicated airspeed as the point where reverse should start reducing during the landing roll, and not 60 knots ground speed?