Reverse Thrust Efficacy
 

Hi all,

Does anyone have a document or reference that gives an approximate number for the efficacy of reverse thrust on deceleration? It is more effective at high speed which we all know but is there a number we could use out there?

I recall on the 737 that using Autobrake 1 and reverse would make the brakes cycle on and off as the rate of deceleration was being achieved by reverse/spoilers and friction and the rate of decel using Autobrake 1 is 4ft per second per second. So I'm guesstimating it's around that rate or slightly less at the nominal touchdown speed of 130-140 knots.

This is obviously a slightly more abstract question and to do with landing performance calculations more than the practical reality usage of reverse which is really another and more simple discussion.

Many thanks

Reverse thrust is most effective at high forward speed. However, idle reverse is effective in removing any residual forward thrust, which may make a significant difference on a slippery runway, even at low forward speed.

I no longer have the reference, but the CF6 on the 747 could achieve a maximum of maybe 40% thrust in reverse. Because of the vector of the reverser airstream, that may translate to ~25% thrust effective.

One thing is for sure. That is you need to read the 737FCTM instructions carefully when it comes to reducing the reverse thrust application approaching taxi speed.

The FCTM states by 60 knots start reducing to reverse idle so that by taxi speed you are at reverse idle which is around 22% N1. Some pilots misinterpret this to mean at 60 knots cancel reverse completely. Nothing could be further from the truth.

Cancelling reverse at 60 knots will cause the aircraft to increase speed momentarily because the engine RPM is still relatively high as it winds down towards 22% N!. In the simulator, prompt cancellation of full reverse (nominally around 88% N1) at 60 knots reveals the reverser lights extinguish by the time the N1 is passing 60% N1 on the way down to forward idle of 22% N1. 60% N1 forward thrust (however momentary) is a lot of power which is the last thing you need if runway length is limited.

Thanks for the replies which reiterate the basic operating philosophy of reverse thrust on modern commercial jets.

However, does any one have any data on rates of decel using reverse thrust alone?

A quick Google search of "reverse thrust deceleration rate" yielded several links, each of which contain relevant data:

http://www.ifalpa.org/downloads/Leve...20aircraft.pdf
https://flightsafety.org/files/alar_bn8-4-braking.pdf
https://ntrs.nasa.gov/archive/nasa/c...9850018449.pdf
Thrust reversers (David Lednicer, Ed Hahn, Robert Dorsett, Terrell D. Drinkard, TriStar500)
https://www.ripublication.com/irph/i...v6n5spl_18.pdf

Efficiency of thrust reverser depends on too many factors to simply list in a document or with an easy equation.
Regarding OP statement in regards cycling of auto brakes there is some truth in this...

Essentially initially the auto rake will ramp up to a preset value, after a time delay which depends on the setting it will 5en increase brake pressure to achieve the commanded auto brake deceleration rate. It continues to monitor the rate and when the rate is achieved it will reduce brake pressure to maintain the rate commanded, if the rate achieved exceeds the commanded rate the brakes do not go back on until the rate decreases below the commanded rate, this causes the cycling as described.

With autobrake 1or 2 selected and the application of 2nd detent reverse thrust the autobrake will not need to apply brake pressure for the majority of the landing roll after the initial ramp up and application, as the reverse thrust and other stooping devices exceed the rate reduction commanded. With an idle reverse landing the continue to apply brake pressure throughout the landing roll at all times.

Hence when a landing with autobrake 1or 2 is made and reverse is still applied when disconnecting the autobrake a shock can be felt as the brakes are manually overridden, as they are not doing anything until the 1100psi is applied to override them. Stowing the reverses to idle and then overriding the brakes ensures a smooth transition as the brakes will be required to exceed 1100 psi by design thus foot brake pressure override is smooth

Good explanation, I avoid that ‘auto brake disconnect
shock’ by easing the speed brake lever forward slightly
this gives a smooth disconnect

On the 787 HUD there is a deceleration rate scale, with marks at the various auto brake settings.

Full Rev, no brakes probably averages out around '3', but starts to taper off later in the landing roll.

Thanks Intruder for the links.

Cough; is that 3 ft per second per second?

It appears from this video that Airbus have something similar but without numbers on their HUD. Skip to 14:00 to watch the touchdown.

The 3 just relates to whatever auto brake 3 deceleration rate equates to. I can't find any info in the FCOM/FCTM to clarify the actual rate I'm afraid!

Intruder has supplied the info you need. See page 5 of the NASA document which shows the Force given by the Reverse fans for a DC10-10.

The Reverse thrust force reduced from around 30,000 lbs at 140 kts to around 10,000 lbs at 20 kts. If you assume an average of 20,000 lbs force on an aircraft weighing up to 421,000 lbs landing weight, then the deceleration due to reverse thrust alone is about 1/21 g i.e. about 1.5 ft/sec/sec or (0.5m/sec/sec).

737-800 HUD had de-acceleraion rate. At touchdown, with max reverse and spoilers deployed, it was approx 2.75. Reduced to 2 at 90-100 kts while still using max reverse.

If you have auto brakes 2 selected you’ll feel the brakes engaging around 90-100 kts as the thrust reverser and spoiler drag no longer provides the de-acceleration required at autobrakes 2.

Judd:
Cancelling reverse at 60 knots will cause the aircraft to increase speed momentarily because the engine RPM is still relatively high as it winds down towards 22% N!. In the simulator, prompt cancellation of full reverse (nominally around 88% N1) at 60 knots reveals the reverser lights extinguish by the time the N1 is passing 60% N1 on the way down to forward idle of 22% N1. 60% N1 forward thrust (however momentary) is a lot of power which is the last thing you need if runway length is limited.

That is because an incorrect technique is used. Reverse thrust should be reduced to IDLE reverse thrust until the engines are at IDLE thrust and then fully cancelled. Snap cancelling will achieve what you surmise. On LST's with cadets, during the mandatory RTO, it was a common thing to see at the 60kts call. They had not been taught properly and their eyes opened, especially when you explained WHY. Too much WHAT & not enough WHY in the teaching.

Fully agree, but WHY takes time my friend and time is money, hence... ;)

In the 747, Approach Idle is active until 5 seconds after touchdown, and when thrust reversers are selected. So even if the engines are allowed to go to idle before deselecting reversers, there will be a momentary (albeit minor) surge forward as minimum idle is selected by the EEC. It can be a real problem on slippery runways, e.g., if attempting a high-speed turnoff and deselecting reverse in the turn.

Other airplanes/engines may be similar.

Approach idle for five seconds after touchdown is standard design practice for all Boeing aircraft (at least the Puget Sound variety - don't know about the former McD Long Beach types). Several years ago the question came up as to the original source of the requirement for a five second delay after the air/ground transition. Best we could come up with was "we've always done it that way":rolleyes:
On the 747-400, approach idle during T/R selection is only the case on the GE CF6-80C2 installation - not so for Pratt and Rolls. This is because the CF6-80C2 uses a pneumatic reverser and approach idle is needed to insure adequate bleed pressure for T/R operation (Pratt and Rolls on the -400 have hydraulic reversers).
The 747-8 has hydraulic reversers and doesn't not select approach idle during T/R selection.

Several years ago the question came up as to the original source of the requirement for a five second delay after the air/ground transition. Best we could come up with was "we've always done it that way

Could it be for that one time, when PF decides with wisdom, that attempting to stop with the little runway left is perhaps not the better option and spooling up and re-launching gives a 2nd attempt to get it right. Remembering the reason why engines are spoiled up first before selecting takeoff power, could this be to avoid any delay in them spooling up for the aborted landing case? Of course, once TR's are deployed all bets are off.

I don't think you can give a single figure for TR deceleration, as it depends a lot on aircraft mass; TRs will be more effective on an empty aircraft.
Wheel brakes are not affected by aircraft mass, as the ground friction increases proportionately with weight, so a set deceleration can be achieved. (up to the point where the brakes become too hot.)
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