B737- Reverse Thrust Operation.
With regard to using reverse thrust after landing the FCTM states (page 6.29)
Maintain reverse thrust as required, up to maximum, 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 to full down after the engines have decelerated to idle...
There is an apparent contradiction where an airspeed approaching 60 knots is the trigger to take action to reduce reverse yet the thrust levers must be positioned to reverse idle by taxi (ground )speed.
Given a 40 knot HW component on landing, it is obvious that at 60 knots airspeed the ground speed will be 20 knots - which is normal taxi speed and by which, according to the above advice, the reverse should be at idle. The reason 60 knots is chosen is presumably because of the danger of foreign object damage and engine surge at high levels of reverse at slow speeds.
This all suggests the speed at which reverse thrust should be reducing is 60 knots ground speed - not necessarily 60 knots airspeed. This becomes more important given stronger headwind components?
Page 2.5 of the B737 FTCM happens to discuss taxi speed and braking and among other points states: Normal taxi speed is approximately 20 knots adjusted for conditions.
In this case it is clear why "taxi" speed is used - meaning naturally, ground speed. This suggests again, that if the reason for given by Boeing at page 6.29 FCTM, for 60 knots as the target speed for reverse reduction (due to danger of engine instability at lower speeds), then certainly a 20 knot ground speed with high reverse exposes the engine to potential damage.
And by my calculation a 60 knots IAS applied to 40 knots HW component means 20 knot ground speed - and that's a bit too low for engine comfort, n'est ce pas?
Moral of the story? If landing into a strong headwind component it may be wise (in order to minimise the potential for engine instability) to start reducing the reverse thrust at 60 knots ground speed - not 60 knots airspeed. Appreciate opinions.