B737 Reverse Thrust Limit

[LEM]

An (unbelievable) doubt emerged yesterday, after our last landing, and as our memory is confused by past -200, NG experience, and today's manuals are so poor in information, we are a BIT confused...:O

The question is: on 737 classic, what is the limit for reverse thrust?

Do we have to be careful not to exceed go-around thrust, or can we pull full aft on the levers?

In forward thrust we can obtain some overboost on the classic, not so on the NG, it seems...

What about reverse?
Is the thrust limited, or can we obtain (intentionally or not) some overboost?

Thankyou.

[XPMorten]

From the manual:

http://www.xplanefreeware.net/~morten/jpgs8/reverse.jpg

Cheers,

Morten

[CaptainSandL]

You can overboost in reverse thrust, I have seen it happen.

From 31 Oct 2004 FCTM, page 6.38:

"After touchdown, with the thrust levers at idle, rapidly raise the reverse thrust levers up and aft to the interlock position, then to number 2 reverse thrust detent. Conditions permitting, limit reverse thrust to the number 2 detent. The PM should monitor engine operating limits and call out any operational limits being approached or exceeded, any thrust reverser failure, or any other abnormalities."

The logic is probably that Boeing are giving you the option to use all available thrust to prevent an overrun say for an RTO, aquaplaning, flapless or generally poor landing.

S&L

[LEM]

Hi XPMorten, that's -200, isn'it?

I'm asking about CLASSICS here, but thanks anyway for your reply.

Hi Chris, yes I carefully checked the training manual also before opening this thread, but it seems to me there is still a lack of a clear and official LIMIT.

And I'm not intersted in passenger comfort here, I'd like to know if pulling FULLY AFT on the levers we get an overboost or if the system is designed so as to limit thrust within the maximum allowable.

The manual states: "Conditions permitting, limit reverse thrust to the number 2 detent."

But it doesn't say WHY.

Is that for passenger comfort? long term engine life? or is that an engine limit?


Also, "The PM should monitor engine operating limits and call out any operational limits being approached or exceeded, any thrust reverser failure, or any other abnormalities."

What are these limits exactly? Are they referring to the 106% N1, 105% N2, 930 EGT?

You see, my question is not so dumb, after all...

Yesterday our Go around N1 was about 93%.... far from the 106% ENGINE LIMITATION.

So, once again, has anybody got a definite answer?
Thankyou. :E

Ps: intersted to hear about NG also...

[XPMorten]

LEM,

Thats from a "combined" Classic/NG manual.

On the CFM56-3, the Maximum Certified
Physical Fan Speed N1 = 106% (5485 RPM) - like you said. Thats for all CFM56-3 engine models.

Depending on what model of the CFM56-3 you have, the Max TO, and Max continuous N1 will differ.
The -B1, -B2 and -C models all have the same core,
e.g. the higher rated -C engine will have a
higher max TO thrust (N1=96%) and the lower
rated -B1 will have a lower max TO thrust (N1=91%) at ISA.
Equally Max continuous thrust is N1=93% and
N1=89% at ISA.
So, I assume that max reverse thrust is similar
depending on engine rating.

What happens between 96% and 106% I don't know. I can only guess that the
engine gets inefficient and life-span gets dramatically shortened and it gets trashed if it has been beyond 106% :p

Cheers,

Morten

[barit1]

Boeing (in olden days) has always countenanced using all the R/T available - to the extent that GE had to put a R/T Limiter device (to prevent overboost) on the CF6-50E for 747-200/-300's.

But aren't -200's (JT8D) still managed to EPR, not N1? If R/T ops state an N1 figure, doesn't that imply the CFM56?

[CaptainSandL]

Hi LEM,

I think you might be trying to look at this a bit too deeply. The engine limits, either absolute (106%) or ambient (Yellow Ref N1 bug on EIS) are the same, regardless of which direction the fan air is being sent.

To put it another way, you can demand as much N1 using the reverse thrust levers as you can by pushing them forward; only the fan air direction is altered and all the limits remain the same.

BTW there are very few dumb questions in aviation. If you were both unsure then you can guarantee others will also be unsure. Better to have an inquiring mind and ask, rather than carry on unaware.

S & L

[lomapaseo]

Monitoring engine limits is the key

However, an accumulation of highpower surges (bangs) will damage the engines in such a way that they may fail days later (cracked blades)

One or two DC9 operators found this out when they used reverser techniques that often popped the engines (too much reverse at too low a speed). Not a problem for any single landing, but the damage will accumulate if you do it for several landings.

[-1000AGL]

XPMorten said:Thats from a "combined" Classic/NG manual.
Could you please expand a bit on which manual that information came from XPMorten?

[XPMorten]

-1000,

It's from an airline that operates both Classics and
NG's. Dated Nov 2002, in accordance with F.A.R 121.141. 2129 pages.
Drop me an e-mail at mmelhuus(at)start.no
if you need more details ;)

Cheers,

Morten

[Red Comet]

For CFM engines, with the PMCs on you can pull full reverse without fear of overboosting. With the PMCs off it is possible to overboost. The limit is G/A N1. As with the -200 if you crank your wrist you get about the G/A limit.
regards

[LEM]

Hi Red Comet,
I don't think the PMC will protect you from overboosting, as the manual clearly states that if you push full forward during take off (as in the case of windshear recovery), you will get overboost.

So, to put it differently:

is the reverse mechanism limited (as suggested by my colleague the other day), or does it allow us to obtain the same amount of N1 as in the forward thrust case?

[Red Comet]

Hi LEM,
Boeing will indeed let you overboost the engines and overstress the airframe - Their philosophy suggests that the operating crew know what a situation requires and will later justify exceeding a limit - when they are safely on the ground.
What I wrote about reverse thrust is true. Take a look at your MEL ref: PMCs inop operational limitations. Should be in there.
regards

[lomapaseo]

While Boeing may allow operation like an overboost by not prohibiting it in the manual, the design of engine FADECs (controller) will not allow it under the regulations.

Even if the manufacturers can not prohibit this operation entirely, the pilots ability to operate outside this condition is like reprograming a computer chip in a highworkload environment.

Of course all the above is mute for discussions about non-FADEC engines like the B737-200's

[Cough]

Having pulled the levers back to max a few of times, the thrust applied on each occasion was short of G/A N1 by 5-10%. As I understand it, it has been designed this way.

-200 - Pull reverse to a certain EPR. Easily possible to overboost.

NG. No idea. Not touched it. Want to though!

[BOAC]

"Having pulled the levers back to max a few of times" - still doing those landings, then, Cough?:D :D

PS Ask for a go in a NG!

Agree with that - never got anywhere near to TOGA reverse on a 'Classic', no matter how far you 'wrist it', whereas as Cough says, the 'Jurassic Classic' was another beast and needed to be watched.

[LEM]

Hi Red Comet, I checked the DDPG under PMC inop, there's absolutely nothing about this matter... :confused:

The fact that pulling fully aft we cannot exceed go around thrust has always been my impression also, but my doubt was about the design of the mechanical linkages guaranteeing this (like FADEC might be able to guarantee electronically)...
or maybe under certain ambient conditions there's still the risk of overboosting...

It would be a wonderful world if the manufacturers provided their pilots with a bit more OFFICIAL information...:rolleyes:

Thanks everybody anyway.

LEM

[Cough]

BOAC

Funnily enough, its the other guy landing when I pull em back...

Still doesn't excuse my own landing technique!

[Ziggy]

B737-800 with CFM56-7B26:

max detent reverse thrust is around 81% N1, while Go Around N1 is typically 90% or more. So to me it would seem impossible to reach an engine limit on the reversers.

There is no extra limitation on flying with EEC's (electronic engine control) in the alternate mode, but dispatch with an EEC inop is not allowed.

Ziggy.