View Full Version : Firewall an NG - what do you get

8th Mar 2004, 12:29
I was just told by a reliable source that if you "firewalled" a 737 NG you got the maximum rated thrust for that engine. That is - even though you had a 24K Engine - whether it was derated at the time or not - if you "firewall" it - you get the maximum thrust, which on the current model CFM56-7B is 27 K (I think).

This is a very misunderstood issue with many pilots - including - yours truly. Therefore I would like a reply from those that have accurate and complete knowledge of this subject - not from reading the Ops Man II, or what you heard happened on the 300 - which I understand is different.

I'm assuming 15C Temp and sealevel - I hope that my question if clear - if not I'll do my best to refine it.

Thanks for any info - I got a bottle of wine riding on this


Flight Detent
8th Mar 2004, 21:32
Hi Loke,
I hope I can answer your Q for you.
When the engine is sold to the airline, the engine control unit is set to the rated power that airline requests, and pays for!

By 'slamming' the thrust levers to full quadrant on an NG, you will not get any more power, the electronics see to that.
It's an unfortunate result of all these electronic systems, in the 'good 'ol days' one could 'firewall' the thrust levers for short periods, with some resulting power benefit, in emergencies, with these systems, one must hit the ground or obsticle with serviceable engines!
Oh, and the highest thrust CFM engines available right now are the CFM56-7B3 or -7B27/A, all at 27,300 lbs @ SL/15C.

Hope this helps, Cheers:cool:

8th Mar 2004, 22:52
n the 'good 'ol days' one could 'firewall' the thrust levers for short periods, with some resulting power benefit, in emergencies, with these systems, one must hit the ground or obsticle with serviceable engines!

Yes, and in the good ole days some guys were failing all their engines by operating them at conditions that they were not designed for. So the regulators required a corrective action.

I seem to recall that the automatic systems now flying some aircraft are also considered an advancement over the good ole days when it was far easier to stall an aircraft out.:O

9th Mar 2004, 00:54
Flight Detent:

Thank you for your informative reply – just the sort of thing I was looking for.

Of course when I say “Firewall”, we wouldn’t be actually slamming the thrust levers forward, but rather gently placing them against the forward part of the quadrant in a very controlled and non-panicky manner.

However, I was told, by someone whose been right on everything before, that the older CFM engines – 737-300 era – were mechanically “pinned” when you purchase them, at the thrust level that you paid for. That when you went full stop with the Thrust Lever – that’s what you got – 22K, 24K – or what ever you paid for originally. The newer CFM engines, however, (700 era) where software limited at your “purchased level”– if you went full stop with these engines – you got the highest rated thrust the engine could produce – apparently 27,300 @ SL/15C.

For example – you are flying a machine that is rated at 24 K with a 26 K bump. If the Bump was NOT selected, I used to assume that if I firewalled it – I got 24 K (despite any assume or derate that I might have selected). According to this new understanding – I would get 27 K – a 12.5% increase over what I thought I was getting. This is good, however comes with some baggage as well Vmcg or mca.

If the above is true – I think there is wide spread misunderstanding amongst the pilots flying this A/C.

BTW – I heard the best definition of assume – it’s the lowest form of human intelligence.

Once again – appreciate your last informative post – if you could clarify my confusion on this issue – I would be appreciative.


9th Mar 2004, 07:51
If you fire wall an NG aircraft you get the maximium thrust for that motor,ie if the CDU Ident page shows you have the following;
CFM56-7b27=27300lbs of thrust at the "firewall".
Take off power settings are only allowed for 5min,Max continous for 10min as per the AMM however you can exceed these provided you DO NOT exceed the 925c EGT limit,exceed the EGT limit and you are dependant upon the amount of temp & time spend in exceedance as to whether you inspect/repair/replace the engine.
You can only buy an up-rate for you engine by purchasing the software disks to reprogram the EEC for the new parameters,change the engine ident plug and provided you also have the correct HMU/fuelpump package as there is service bulletins listing the requirements.
The FDAMS are checked daily and are used for trend monitoring/ETOPS reliability data.The FDAMS takes a "snap shot" of all engine parameters at Takeoff,Stable cruise,and postflight .At the company i work for,All this data is sent to CFMI in the USA who track the data and advise the operator of any major problems upon discovery,reliability reports are sent regularly.

Flight Detent
9th Mar 2004, 11:04
inappropriate comments deleted

The electronics are there mainly because the current two pilot cockpit can operate the engines (and many other things) without paying the amount of attention to them they deserve, because they have to distribute their attentions throughout the cockpit, since there is no FE on board, who normally looks after all these things very efficiently.

Regulators indeed!

I guess when anybody is asleep at the controls, regardless of the age of the airplane, the stick shaker will awaken them!! (if the FE is having a break)

inappropriate comments deleted

11th Mar 2004, 01:12
Mates, to clear up the fog in my mind I need a brief resume:
are the following statements correct?

737 Classic, 20k nominal thrust, ISA sea level: at the TOGA setting I get 20k, if I firewall it I get more, example 22k.

737 NG, 27.3k nominal thrust, ISA sea level: at the TOGA setting I get 27.3k, if I firewall it I still get 27.3k due to FADEC.

11th Mar 2004, 23:50
I'll chime in. On the -700 dash power takeoff, I nearly firewall the throttles to get the 22k we paid for, even though software changes to come will give us 24k. So that tells me you get what you pay for. I can't get the power above the N1 setting the CDU tells me to set no matter how far up I push the power.

13th Mar 2004, 19:45
The discussion above started me thinking what it costs to 'buy the higher engine rating' and to operate it? I'm working a bit in the dark here, and if anyone has better figures, they would be welcomed. The cost difference probably comes down to fuel, maintenance and capital.

I'm guessing that the difference in fuel consumption between the different motors in service must mainly be at takeoff, since they have the same thrust at cruise at altitude. SNECMA data on max. climb thrust @ 35 000 ft and Mach 0,8 and ISA and engine installed, for all ratings, is 5,960lbs.

I have no idea if there is a difference in the fuel used in climb. Any inputs welcome. I estimated the difference in fuel use between engines with different ratings assuming a constant SFC of 0.36 lb/hr/lb and no derating on takeoff. If the difference in fuel use between the different ratings can be simplified to 5 minutes takeoff thrust (and I'm not sure if this approach is valid), then the extra fuel burn per flight based on US$1.30 per US gallon (about AUD$0.45 per litre), is

CFM56-7b18 19500lbs US$0 base case
CFM56-7b20 20600lbs $6 extra over base
CFM56-7b22 22700lbs $18 extra over base
CFM56-7b24 24200lbs $ 27
CFM56-7b26 26400lbs $ 40
CFM56-7b27 27300lbs US$ 45

It costs more to maintain the higher rated engines. The higher engine ratings go hand-in-hand with a lower EGT margin, and that generally means that the shop visit comes up earlier. The best I could find on the CFM engine data is:

CFM56-7b18 19500lbs 145
CFM56-7b20 20600lbs 145
CFM56-7b22 22700lbs ?
CFM56-7b24 24200lbs 125
CFM56-7b26 26400lbs 85
CFM56-7b27 27300lbs ?

I have better data for the V.2500 motor. The issue of workscope at the shop visit is complex (sounds like the mechanic at my local garage, only he uses shorter, more punchy words). But it comes down to 19,000 hours on the wing use before the first visit for the 22,000lb motor and 12,000 hours for the 30,400lb motor. The maintenance varies of course by the ratio of engine flight hours to engine flight cycles – this data is based on 2 EFH/EFC.

The run to the second shop visit is about 70% of that. Engines with average cycle times around 1 EFH will only achieve 12,000 EFH because most damage is being done at takeoff. Hot environments also worsen the wear. At the lowest engine ratings though, the deterioration rate of EGT margin per 1000 EFH is pretty low, and its other factors cause removals rather than EGT margin. That gets complex to model and I've left out its discussion here; also there is no published discussion about how often they replace the spark plugs, but I suppose that must be buried in the calculations somewhere.

Anyway, the cost per flight hour for the different ratings can be calculated as:
Model V2522 V2527 V2533
Thrust rating 22,000 26,600 30,400
EGT margin 90-115 70-80 40-60
$US/EFH $103 $122-131 $150-161

So it costs about $30-50 per flight hour in extra maintenance for the higher rated engines. The CFM data indicate more EGT margin at higher power ratings than the V2500, so the difference in maintenance costs might not be as great for the CFM 56.

I have no idea at all on what the different ratings cost, but my suspicion is that the fairly simple process to uprate them, outlined by DDG above, doesn't mean they are cheap to buy. I'm guessing here, and wonder if anyone has better figures, but they probably price the upgrade in terms of $/lb thrust. So if the engine cost about $4 million for say 20000 lbs thrust, then that is $200 per lb, and an uprate of 1000-2000 lbs might be charged at something like the $US 200,000 level. Or the 27,300 lbs rating engine might cost some $800,000 more than the 19,500 lbs rating. Anyone got more information?

For an aircraft doing 3000 flight hours per year and 1500 flight cycles, I estimated that changing from the 22700lb motor to the 26400lb motor costs (in US$ per year terms):
Maintenance $90,974
Fuel $31,928
Capital cost $26,600 (actually increasing lease costs)
TOTAL $149,502 per motor per year.

In a 30 aircraft fleet, with 2 motors per airframe, that's about $9 million per year.

14th Mar 2004, 19:06
The only source documents that I've been able to locate on the subject of what do you get when you firewall the NG is from Ops Man Vol II - Engine sec covering the EEC:

The full rated takeoff thrust for the installed engine is available at a thrust lever position less than the forward stop. Fixed or assumed temperature derated takeoff thrust ratings are set at thrust lever positions less than full rated takeoff. If the thrust lever is advanced to the forward stop, the EEC limits thrust to the maximum thrust rating offered for the airplane model.

The standard reserve thrust rating available at the forward stop varies according to fleet configuration as follows:

• CFM56-7B24 rating

Unfortunately this can be read 2 ways. The - normally very reliable guys I'm talking to - say that the first part claiming that you get "maximum thrust rating offered for the airplane model" means that you would get 27 K no matter what engine you paid for. The following lines specifying a destinct engine seems to contradict that - Clear as mud.

Does anyone have any other source document type proof - I've got a bottle of wine riding on this so clearly the proof has to be in the region of a Capital 1 Murder case.


14th Mar 2004, 20:36
Out of interest this is what our manuals say about the classic:

The 737-300 has a significant thrust overboost capability which
could possibly be used to advantage in emergency situations. This
overboost capability should only be considered during emergency
situations when all other available actions have been taken and
ground contact is imminent. Overboosting the engines when the
situation is not sufficiently serious, or prior to performing all other
recovery actions available, would unnecessarily increase the
potential for engine failure.
In an emergency situation 'firewalling the thrust lever' should be
considered. This condition could result in an EGT or N1 over the
red line.
The aeroplane must be landed at the nearest suitable airport
following a "firewalling" thrust overboost.

I cannot copy the text for the NG but it is very similar to the previous post. The key part says:

If the thrust lever is advanced to the forward stop, the EEC limits thrust to the maximum certified thrust for current conditions.

15th Mar 2004, 15:49
Sorry mate, but I'd rather clip the top of a forest, climb away and lose an engine to "operating them at conditions that they were not designed for", than go four wheel driving through the countryside with two 'perfect' CF6's.
Not that EEC's are a 'bad' thing, its just that when the sphincter factor starts to jump, i want the donks to get in there with all they got!
I'm sure the company wouldnt even mind payin to dryclean my underwear after the incident! :eek:

17th Mar 2004, 14:03
May I chime in here and ask a (hopefully) unambiguous question to clarify this, as I am interested in the answer also.

A) Company pays for a 27k bump engine with 22k 24k 26k derates.

1. Takeoff at fixed derate 22K. Firewall, what do you get?

2. Takeoff at 24K assumed temp. Firewall, what do you get?

B) Company pays for a 24K engine. Takeoff at 22k assumed temp. Firewall, what do you get?

I could try it in the sim soon, but perhaps someone has the horse's ear on this.

17th Mar 2004, 17:57
Let me check if I got it wright (although not operating the NG):

A) Company pays for a 27k bump engine with 22k 24k 26k derates.
1. Takeoff at fixed derate 22K. Firewall, what do you get?
22k only, because of the FADEC.

2. Takeoff at 24K assumed temp. Firewall, what do you get?
27K - or slightly less if the FADEC thinks it's very hot.

B) Company pays for a 24K engine. Takeoff at 22k assumed temp. Firewall, what do you get?

Waiting to be shot down...

17th Mar 2004, 20:59
RAT 5 -

Is there a way you can measure "thrust" in your simulator?

Flight Detent
18th Mar 2004, 02:30
Hi all,
You may want to consider selecting the EECs to alternate, that should encourage it to give whatever it's got!
That is, within the thrust rating it's rated for, e.g., if it's a 27K engine, it may give you slightly more in a real emergency! - and the same for other rated engines, just slightly more!
(With the accompaning EGT and N1 exceedances, of course!)

I guess all the dials will be red when you get to destination!!!


18th Mar 2004, 02:31
It is so simple ,if you firewall a B737NG the thrust produced will be the max rated that you bought as shown on the CDU IDENT page subject to outside atmospheric conditions.
If you have a 22k engine the most you will get is 22700lbs as the EEC will use a fuel schedule to govern N1 speed to produce a N1 speed that equates to 22700lbs.
This max speed of N1 may be reached prior to the thrust lever reaching the cockpit fwd stop but the EEC will not schedule any more fuel,therefore you will not gain any more thrust with further thrust lever movement.

18th Mar 2004, 11:57
From the XYZ Airline Volume 2.

"The full rated takeoff thrust for the installed engine is available at a thrust lever position less than the forward stop. Fixed or assumed temperature derated takeoff thrust ratings are set at thrust lever positions less than full rated takeoff. If the thrust lever is advanced to the forward stop, the EEC limits thrust to the maximum thrust rating OFFERED for the airplane model

The standard reserve thrust rating available at the forward stop varies according to fleet configuration as follows:

aircraft registrations 1 - CFM56-7B24 rating
aircraft registrations 2 - CFM56-7B27 rating

For those that are not in XYZ Airline, registrations 1 are 700's with a mix of 20k and 22k motors, and registrations 2 are 800's with a mix of 24k and 26k motors.

so to answer the question, firewall the levers in a 700 and you get 24k, in a 800 you get 27k (assuming ISA +15 or less and sea level)

LOKE, I appreciate that this is essentially your post reproduced, however the addition of the two types helps clarify the mystery....

Please don't cite and identify the company ...

have another coffee
18th Mar 2004, 13:18
And we have a winner!
Dehavillanddriver is right.
firewall a NG and you get:
-BBJ=26K :ooh:

Note; there is NO egt protection whatsoever (except when starting an engine on ground). Only N1 and N2 are protected by the EEC (in normal mode).

edited to ad note

19th Mar 2004, 10:20
Just a quick question from a ‘Bus’ driver for you NG guys.

Can you put a fixed derate into the perf page of the MCDU?

If you can, then I would be surprised if you were able to achieve the normal max thrust by ‘firewalling’ the thrust lever. If you did firewall the donk with an engine failure on the runway above V1 when using T/O figures for a fixed derate, and did actually achieve the full thrust, then you would be in danger of losing control of the aircraft because it could well be below the VMCG for full thrust.

19th Mar 2004, 11:07

Yes you can put in a fixed de-rate, on our 26k aeroplanes you can use derate -1 which is 24k and derate -2 which is 22k.

The Vol 1 says that rated thrust is achieved at a thrust lever position less than the forward stop - ie there are more n1's available if you poke the lever forward.

V1MCG isn't a huge issue, even at 26k it is only around 108-110 knots.

you would have to be pretty light to be using speeds that low.

20th Mar 2004, 02:54
One of the benefits of using a fixed de-rate is being able to lift a greater weight off very short runways when you are 'relatively' light.

In a -500 TriStar for example we could lift up to 20T more off short runways because we were derating to a 22B thrust instead of the full 524 power. This amounted to some 8000lbs less per engine. Admittedly the short fuselage -500 was quite Vmcg limited, but even so I am surprised that full power is available when using a fixed de-rate.

This is not the case on the 'bus'.

20th Mar 2004, 03:46
.. a few comments

(a) lomapaseo was pushing a sensible risk management line - the electronics in modern aircraft may not save the day every time (recent history indicates, quite clearly, that they don't - what's that you say ? our PCs have bugs as well in the software ?) but they can prevent the ham-fisted pilot from creating more problems than he/she might solve by being innovative. It is a question of maximising the probability of a satisfactory outcome .. no guarantees only consideration of risk minimisation and probabilities ....

(b) one poster suggested that he/she would prefer to clip the tree tops, overboost and climb away .. that's fine ... but the risk is that you might also overboost, lose it, and impact wingtip first .. which scenario do you prefer ? .. a controlled impact or blazing saddles cartwheeling ? I guess one might prefer the "die with his boots on" attitude .. but the problem is far more complex than one of the simple "I'll save us ... " philosophy.

(c) I note that Flight Detent was somewhat impolite in his/her response to lomapaseo. Please, good folk, can we keep posts in this forum focussed on the topic and leave the colour to other, more appropriate, forum threads. Here we play the ball ... not the man.

(d) if you predicate a takeoff on a derate configuration (as contrasted to reduced thrust) and then push up the levers, especially at low weights and speed schedules, you expose yourself to Vmcg/Vmca problems. If you haven't been there (and are not familiar with the VERY rapid onset of divergence with many aircraft) and are not trained to react appropriately rather than in panic ... I suggest that you consider not pushing the levers up other than VERY, VERY cautiously. Do keep in mind that it is not just a case of being near the published Vmcg/Vmca but that an increase in thrust may result in a considerable INCREASE in the realworld Vmcg/Vmca. It is a little disturbing to read what appears to be a gung-ho approach to this consideration in many of the posts in this thread ?

(e) Overrun's post makes for interesting reading ...

(f) the main concern, I suggest, is one of turbine temperature (and gearbox loads in the case of turboprops). Modern turbine blades are thermally critically loaded and, in cases of growth engines, don't necessarily have much fat for the pilot to play with ... you over temp such animals at your peril .. and the consequences may not be very long in following the overtemp incident. I am aware of one recent investigation where, with such an engine, an apparently innocuous overtemp did considerable crystalline damage to the blades ... and cost the operator a fistful of money to rectify.

(g) dehavillandriver ... please don't cite the source of confidential documents .. XYZ management might not appreciate your quoting from company confidential documents in a public forum without authority to do so .. if you must so quote, then please leave out the citation ...

23rd Mar 2004, 05:24
John Tullarmarine.

The regos and thrust ratings are public domain - just go and check the CASA and or ASA (avcharges) website if you are so concerned.

The quote from the manual is not company specific, so I accept your point, however I see no reason to cite the company name - company names are attached to posts alleging all manner of thing, most of them unflattering, with no editing.

The use of a company name assists in putting the information into context.

23rd Mar 2004, 11:31
Good friend, DHD,

Sorry to be pedantic .. but no company identification, please, (whether direct or indirect) where such identification may

(a) bring the site or this forum into disrepute

(b) lead to the occurrence of calls in relation to the items from legal people to Danny et al on the top floor. I note that such occurrences are likely to be accompanied by follow-on calls from Danny et al to the moderators of this forum and, now that we chaps are getting older .. we really need our sleep).

Forum policy as in the sticky.

Not negotiable in this forum - period.



(modified for clarification after sleeping on it for the night - without phone calls from Danny)

24th Mar 2004, 18:01
- 700 22K firewall you get 24 K + something
- 800 24k firewall you get 26K + something
- 900 26K firewall you get 26 K + something
the something is what keeps the engine from exceeding EGT limits