737 reverse thrust effectiveness at Detent No 2.
Thread Starter
Join Date: Apr 2005
Location: Australia
Posts: 1,414
Likes: 0
Received 0 Likes
on
0 Posts
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?
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?
Join Date: Oct 2009
Location: UK
Posts: 1,270
Likes: 0
Received 0 Likes
on
0 Posts
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?
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?
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.
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.
Join Date: Jul 2000
Location: UTC + 5.30
Posts: 50
Likes: 0
Received 0 Likes
on
0 Posts
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.
It is always better to use maximum reverse on wet,slippery runways and on short fields.
Join Date: Jun 2006
Location: Australia
Posts: 1,186
Likes: 0
Received 0 Likes
on
0 Posts
but the cost of a new set of brakes is astronomical when compared to the fuel usage during moderate reverse thrust usage.
Mistrust in Management
Romeo E.T.
but the cost of a new set of brakes is astronomical when compared to the fuel usage during moderate reverse thrust usage
To an extent I also agree with Captjns' tongue in cheek comment
The function of Thrust Reversers on the CFM equipped 737 is to make lots of noise and consume fuel.
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
The function of Thrust Reversers on the CFM equipped 737 is to make lots of noise and consume fuel.
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.
Join Date: Jun 2006
Location: Australia
Posts: 1,186
Likes: 0
Received 0 Likes
on
0 Posts
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.
Join Date: Nov 2007
Location: The Colonies
Age: 47
Posts: 7
Likes: 0
Received 0 Likes
on
0 Posts
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.
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!
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!
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.
Per Ardua ad Astraeus
Join Date: Mar 2000
Location: UK
Posts: 18,579
Likes: 0
Received 0 Likes
on
0 Posts
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!).
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!).
Last edited by BOAC; 26th Mar 2010 at 16:54.
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.
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
The latest FCTM (Oct 31, 2008) has various references to this but the following are probably most applicable:
Reverse Thrust Operation (Page 6.48)
“After touchdown, with the thrust levers at idle, rapidly raise the reverse thrust levers up and aft to the interlock position, then to the number 2 reverse thrust detent. Conditions permitting, limit reverse thrust to the number 2 detent.”
“After touchdown, with the thrust levers at idle, rapidly raise the reverse thrust levers up and aft to the interlock position, then to the number 2 reverse thrust detent. Conditions permitting, limit reverse thrust to the number 2 detent.”
Minimum Brake Heating (Page 6.48)
“• select maximum reverse thrust as soon as possible after main gear touchdown. Do not wait for nose gear touchdown. The intention is to use reverse thrust as the major force that stops the airplane. The use of maximum reverse thrust further minimizes brake heating”
“• select maximum reverse thrust as soon as possible after main gear touchdown. Do not wait for nose gear touchdown. The intention is to use reverse thrust as the major force that stops the airplane. The use of maximum reverse thrust further minimizes brake heating”
S&L
Per Ardua ad Astraeus
Join Date: Mar 2000
Location: UK
Posts: 18,579
Likes: 0
Received 0 Likes
on
0 Posts
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'.
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'.
