737 reverse thrust effectiveness at Detent No 2.
 

Discussion arose on the CFM 56's in the 737 and use of reverse thrust after landing. The FCTM mentions "Conditions permitting, limit reverse thrust to the number 2 reverse detent".

Some operators prefer to mandate full reverse thrust on all landings. Often the No 2 detent can be hard to detect during reverse thrust operation.

It is a fact that reverse is most effective at high speed reducing in effectiveness below 80 knots. What N1 can be expected at the reverse No.2 detent compared to full reverse which is up around 88-90 percent N1?

My guess is around 60-70 percent N1 at reverse detent No 2 but I could be a long way out. With that relatively low reverse thrust level at the No 2 detent, would it be true to say the difference in rate of decelleration between full reverse and detent No 2 at 70 percent N1 would be halved or thereabouts. Put another way, reverse detent No 2 is no way as effective at high speed as full reverse?

Hi,

Don't know about your 737s, but on our CFM A320 / 319s unless the runway is slippery, or the procedure requires full reverse, we routinely only use idle reverse on landing and auto brake. More reverse keeps the brakes cooler but makes lots of noise. The stopping position would be identical, because the autobrake modulates to maintain a predetermined decel. rate.

How short and slippery are your runways?

I think that you will find that detent 2 is very easy to find - you are probably mistaking detent 2 for "full reverse". Full reverse is up and back on the reverse thrust levers after detent 2 - and the levers only move by about 1.5 cm from the detent 2 position. :8

Maximum reverse thrust and N1 is from 80-84%. One detent lesser than this and N1 is from 72-75%.These are approximate values.
It is always better to use maximum reverse on wet,slippery runways and on short fields.

The function of Thrust Reversers on the CFM equipped 737 is to make lots of noise and consume fuel.

but the cost of a new set of brakes is astronomical when compared to the fuel usage during moderate reverse thrust usage.

Not to mention the heat build up as the brakes absorb more stopping energy required when less than full reverse is used. With the tendency for quick turn arounds in low cost carriers, that heat has little time to dissipate by the time the next take off run commences. That leaves an unknown amount of already stored heat in the brakes in addition to the energy absorbed if a high speed heavy weight abort just happens to occur. Use of auto-braking as well as reduced reverse thrust (as against full reverse) on landing, all adds up to unnecessary brake heat build up which the next pilot may cop to his cost on the next leg. .

Romeo E.T.
 

That may be so but studies carried out by at least one large airline indicate that the cost of more frequent engine hot end maintenance (due to additional heat cycles generated through use of reverse) easily offsets the cost of brake usage. Also if you have carbon brakes the extra wear for the additional braking effort is negligible.

To an extent I also agree with Captjns' tongue in cheek comment .

If runway length and/or braking effectiveness is in doubt then of course use up to full reverse. However 3000 meters of dry runway and I sometimes see folk apply full reverse down to 60 kts - the pax don't like it and nor do the beancounters.


Regards
Exeng

Actually they are pretty effective. This (incorrect) attitude comes about because the autobrake schedules a set deceleration rate - so when you hammer the reverse, the autobrake reduces the braking effort. The pilot feels the same deceleration with the reversers roaring and assumes that the reversers aren't doing anything.

Next time you are on a decently long runway, land with the autobrake off, your feet OFF the brakes, and use full reverse to see how quickly you pull up.

We do this demo in the 737 Classic simulator whereby conduct max structural take off and 11,000 ft runway. At V1 of 150 knots, the take off is rejected using only spoilers and full reverse. Every time the aircraft leaves the end of the runway around 45 knots in nil wind. The deceleration with full reverse and spoilers is impressive at high speed, reducing markedly below 80 knots until with full reverse still applied the deceleration is minimal. However it is on a slippery runway when the reverse comes into its own because auto or manual braking efficiency is greatly reduced and early use of full reverse cuts the speed back while the brakes are struggling due slippery surface. Different story on the bucket reversers of the 737-200 which are really effective down to a stop - but terribly noisy and lots of buffeting of rear fuselage which doesn't impress the passengers.

I'm surprised noone has addressed the SWA overrun accident at MDW. Those gentlemen landed right in the TDZ, on-speed, with MAX autobrakes, and departed the end solely due to failure to use reverse in a timely manner. Almost nobody at SWA realized that 737ng stopping distances were predicated on reverse usage with contaminated runways.

So although the trend has been idle reverse and heat the (carbon) brakes, being familiar with when/where max reverse is vital is another important tool in the kit.

Also whilst we're on the subject Boeing FTM says select idle reverse on main gear touchdown and the rest (detent 2 or max) when the nose gear touches.

I see some pilots selecting reverse thrust much too late! Also if you're using a/b 3 or MAX by the time you've thought about it the nose gear is on the ground so it's best to think of it as one continuous selection.

Am all for economy and reduction of noise etc. but Boeing put Flaps 40, autobrake (backed up by manual brakes) and reverse thrust on the a/c for a reason - if you need it use it!

My B737 FCTM (October 31 2006 page 6.28). is obviously out of date as the above advice does not appear. Request details of your quote - date of FCTM - B737 NG or Classic etc. Are you sure what you have stated is not a company insertion in the FCTM because by the time you are through the interlocks the nose gear is already on the ground with normal de-rotation rate so you wouldn't have time to get into idle reverse before you haul back further into full or detent 2.

Cent - I'm not sure if it is in later FCTM than mine (Oct 2007) but I was always very cautious about this, and I would rather see positive confirmation of spoilers BEFORE deployment than a panic haul on the levers.

9 times out of 10 the reverse has no effect on landing distance with autobrake and so many F/Os were being driven by the trainers into rapid deployment in my last company that their fingers were often creeping forwards on the T/Ls during the flare and one guy actually deployed them while we were still airborne.

Shades of Conghonas? Granted the Boeing still allows you to deploy the levers, thank the Lord!

EDIT: For aewanbe - all autobrake landing figures in the QRP include 2 engine detent reverse with additions for non-availability (always a mystery to we drivers!).

CFM 56 comparitive thrust level. 20K engines. Sea level ISA. What is the thrust amount at (say) 23 percent N1 (idle).
55 percent (typical short final)
72 percent N1.
88 percent N1?

Thanks.

Firefly, Cent, BOAC,

The latest FCTM (Oct 31, 2008) has various references to this but the following are probably most applicable:

Putting these two statements together it appears that the Boeing intention for a non-min brake heating landing, ie a normal landing, is for a two stage application of reverse thrust. In practice few people consciously do this, but since the sleeves take a finite time to travel I doubt that there is anything to be gained by the two stage technique.

S&L

That is no change from the previous edition. In my opinion folk are mis-interpreting the words so that it becomes a matter of 'life and death' to whack in the reversers the moment the mainwheels grease the tarmac. As far as I am concerned, it means do not delay un-necessarily - unless of course you are landing on a flooded or icy limiting runway or you need every ounce of reverse stopping force available. On 99/100 landings it makes little difference. (You could probably actually multiply that fraction by a few factors of 10!).

I had one 'keen' trainer tell me I was not selecting reverse 'quickly enough' on an 8000' dry runway with autobrake 2. We disagreed.

The 'older' 737 drivers will recall that the limitation on reverse deployment with the nosewheel off the ground used to be to protect the buckets. I think even that changed in my time on the 200 to 'not before de-rotating'.

Thanks for the correction guys!