B737 Reverse Thrust use during aborts and landings

[Tee Emm]

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?

[pattern_is_full]

While you make a fair point, my guess is simply that AIRSPEED is the speed by which flying operations are conducted.

It is the BIG number in the instrument scan - much easier to see, while trying to maintain control of an aircraft, than a GS or wind indicator over in the corner of a flight display. It is THE number drilled into pilots as the critical thing to watch.

Especially in an abort, I'd say asking a pilot to divert any amount of attention to searching for a minor GS gauge, or doing math in their head ("Hmmm, we are doing 90 kts on the big gauge, but the headwind (the last we heard on ATIS from 20 minutes ago) was 30 kts, so our GS is now 60 kts and we can kill the reverse.") when there are bigger fish to fry (like stopping the plane before the hard stuff runs out) is inadvisable.

And I'd bet that is Boeing's reasoning, also. The KISS principle.

I guess a PIC could include such calculations in briefing a potential RTO or landing ahead of time, while calm and collected.

[DaveReidUK]

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)

I would also suggest that the risk of FOD from ingesting debris kicked up by the reversers is considerably greater at 30 kts in still air than at 30 kts GS into a 30 kt headwind i.e. 60 kts IAS - hence the IAS limitation.

[sheppey]

It is the BIG number in the instrument scan - much easier to see, while trying to maintain control of an aircraft, than a GS or wind indicator over in the corner of a flight display. It is THE number drilled into pilots as the critical thing to watch

Fair comment, but I guess it depends on the type of display. In its discussion on erroneous airspeed, the B737 FCTM under Non-Normal Operations section, discusses flight instrument displays and airspeed unreliable. Among other things, unreliable airspeed indications can result from blocking or freezing of the pitot/static system.

If the opportunity arises ask the simulator instructor to set up a blocked captain's static vent on take off. Shortly after lift off, not only will his altimeter not show a positive rate of climb but the airspeed indicator reverses and slowly goes backwards triggering windshear warnings and eventually a stick shaker. It can be thoroughly confusing to the unprepared which is why it is such a valuable training exercise in the simulator.

As the FCTM states, if the crew are aware of the problem, flight without benefit of valid airspeed information can be safely conducted and should present little difficulty. In addition, ground speed information is available on the instrument displays assuming you know where to look for it. If you don't know where to look for it instantly, you should not be in the job.

For all the reasons given in the FCTM, it is a good idea before starting the take off roll to know the headwind or tailwind component. Not only is it a good idea but a vital aspect of good airmanship. Thus, if a 20 knot headwind is present on take off, it takes only a fraction of a second to glance at the ground speed reading at the 80 knot or 100 knot airspeed call and know that the ground speed should be roughly 20 knot less than the airspeed.

Now, if a significant airspeed discrepancy is noticed between the PF and PNF ASI readings at the 80 knot or 100 knot call, and there is momentary uncertainty which one is correct, a glance at the ground speed indication will offer a clue. if necessary, the pilot can fall back on the ground speed display to take whatever action is appropriate. Normally it is safer to keep going rather than risk a high speed abort.

[Skyjob]

The answer is actually quite simple:

Airborne - use airspeed, it's the speed which keeps you aloft, makes you fast enough for take-off etc.

On ground (during landing or RTO) - use ground speed, as this is the speed used to determine the kinetic energy which needs to be absorbed from a moving object to one standing still. Remember the 1/2 * Mass * Vground squared, the latter being read from the GS indication?

During a stop, any stop, the object/aircraft requires a deceleration and this is done using (kinetic) energy dissipation, through use of various methods and factors. These include and are not limited to spoilers, flaps and airframe providing aerodynamic friction, brakes providing either a predetermined deceleration rate or a maximum pressure on brakes resulting in a stopping force, and last but definitely not least the reverse thrust.

If reverse is not used, the brakes needs to do more and as a result get hotter, thus needing more cooling afterwards, etc... Hence the two tables for cooling, with and without reverse in chapter PI.

Recommendation is thus to ALWAYS include GS in your stopping actions. This can be done by remembering the GS during RTO only as used in some companies. Or much more practical to call groundspeed out loud as a function of the "speedbrake up" call, eg "speedbrake up, 145". The latter would allow either pilot to hear the GS thus hopefully remembering it for potential cooling calculations afterwards. This should especially be done in airlines who operate short turnarounds. Its importance cannot be overestimated.

Airborne = IAS
Ground = GS

[pilot-737]

Aerodynamically there is no difference for your engine if you are standing with a 60 kts headwind or rolling at 60 kts with 0 kts headwind your engine's (intake) speed relative to the air mass is exactly the same !and that relation determines your FOD ingestion / stall risks ! Your ground speed is not relevant in this case !
Imagine an example with a 20 kts tailwind RTO ... If your ground speed is 60 kts your engine's "airspeed " is only 40kts the same as your pitot reading do you think that is appropriate to use your ground speed to cancel your full reverse thrust in this case ????
In case of rejected take off as long as the correct wind is taken in to account when entering your t/o performance tables once again your IAS is your reference speed because your V1 (indicated) takes in to account your ACTUAL ground speed with your CURRENT V1 with a safety margin of course (Only 50%of your headwind).

[de facto]

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.

True,reverse is not that effective at low speed as the autobrakes are designed to decelerate the aircraft at the preset value chosen by the pilots.
With the autobrakes,the reverse thrust simply reduces the brakes force to achieve the preset decelration.
HOWEVER,if the pilot elects to disengage the auto brakes,the reverse up to speed 0 is additive to the stopping force using manual braking.

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.

Please dont forget your brakes temperature in your equation...

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.

It could indeed,the Engine Temp will be higher as less air into the compressor,the lower the speed the bigger chance of FOD being sucked in(eventhough runways should be inspected regularly).
The vortex created by thrust on the CFM is reduced with Headwinds,10 kts HW will destroy the vortex up to the usual taxi max N1 of 40 % and 30kts HW will destroy the vortex up to max take off N1.
So good airmanship dictates to follow boeing recommended practices including their fctm on use of reverse thrust which is at 60 kts IAS,reduce the reverse,to be cancelled by taxi speed.
PF should not rely on a call out to start such an action,but by also scanning his instruments just as one would do when the VR call is made.
When braking action is less than good,reverse can and should be used until taxi speed is achieved and or if runway limited until a full stop is achieved.

The ingestion of potential fod is now not the important factor,not being in an overrun is.
The Variable Bleed Valves are designed to reduce the possibility of engine fod damage during reverse ops.

[Cough]

Pilot 737, has it.

It's not just about FOD, but also about recirculation of exhaust leading to stall/surge. In a TW, if you left it to 60 GS that may occur.

[KAG]

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.

If you speak about the ingestion hazard, the relative wind, so indicated speed is the important one, not the ground speed.

[300-600]

Consider also the runway state. You say reverse thrust is not that effective below 80 knots... in the aquaplane / ice / contaminated runway it is more or less all you have got. Our SOP is to keep reverse idle until taxy speed as it takes around four seconds to spool up to full reverse from this contition (as opposed to 12 seconds if you have retracted them fully). This was reinforced by an incident where the reversers were stowed on a wet runway at 60 knots IAS (10 knot tailwind) and heavy rubber deposits at the upwind end put the brakes in a virtual "black ice" situation. It took so long to get the reversers back out they came very close to the grass.

[sheppey]

With the autobrakes,the reverse thrust simply reduces the brakes force to achieve the preset decelration

With RTO position selected the autobrakes slam on full available brake pressure.

[sheppey]

in the aquaplane / ice / contaminated runway it is more or less all you have got

Agree. In the simulator it is surprising to see how well the reversers help in the overall braking on a slippery runway.

[de facto]

With the autobrakes,the reverse thrust simply reduces the brakes force to achieve the preset decelration
With RTO position selected the autobrakes slam on full available brake pressure.

True,i had in mind the autobrake in the landing mode not RTO,in the latter case full hydraulic pressure is used.
Thrust reverse use in that case adds to deceleration just as during manual braking.

Now, if a significant airspeed discrepancy is noticed between the PF and PNF ASI readings at the 80 knot or 100 knot call, and there is momentary uncertainty which one is correct, a glance at the ground speed indication will offer a clue. if necessary, the pilot can fall back on the ground speed display to take whatever action is appropriate. Normally it is safer to keep going rather than risk a high speed abort.

How about the stby instrument?

[Skyjob]

Each Autobrake Selector Setting has a Maximum Pressure at Brakes (PSI) and a preset Deceleration Rate (ft/secē) except RTO, which is uncontrolled maximum pressure.
The following table applies to 737:

Autobrake 1 - Maximum 1250 PSI, Deceleration Rate 4 ft/sec2
Autobrake 2 - Maximum 1500 PSI, Deceleration Rate 5 ft/sec2
Autobrake 3 - Maximum 2000 PSI, Deceleration Rate 7.2 ft/sec2
Autobrake Max - Maximum 3000 PSI, Deceleration Rate 12 ft/sec2 (below 80kts)
Autobrake Max - Maximum 3000 PSI, Deceleration Rate 14 ft/sec2 (above 80kts)
RTO Full Pressure, Not Controlled