B737 NG Certified/Rated/Derated thrust
 

Hello folks, the company I work for recently began using double derates, so a fixed soft derate and assumed temperature together. This has led to some confusion in certain cases as you might imagine.
Colleagues have told me that they prefer not to derate because 'in windshear and go-around you will not get maximum thrust anymore'. The manuals aren't terribly clear on this, I checked the AMM-SDS as well but couldn't find a clear statement concerning this. Here is my understanding of the thrust ratings (Please correct me!)

Firstly the CFM-567B has a maximum certified thrust of 27.3Klbf - well, if N1 and N2 stay below redline - can any rated engine achieve this thrust when fire-walling the thrust levers? AMM SDS 73-21-00 mentions six N1 reference speeds, one of them is Maximum takeoff/go-around at 78°TRA (which I assume is based on engine rating) and also mentions maximum certified thrust at 82.5°TRA. Would that be 27.3K?

The identification plug will determine the hard derate (so the maximum thrust the EEC will provide voluntarily) and thus the maximum thrust available on the FMS at TO. Also this will dictate the thrust available when double clicking TO/GA.

The FMC will allow selection of a fixed/soft derate and an assumed temperature. Depending on the pins set the thrust limit displayed might be either only the TO/TO-1/TO-2 fixed derate thrust or the double derated derate+TASS thrust. Am I right in my understanding that this will not limit you on subsequent go-arounds or during windshear recovery (but instead the maximum rated thrust set by the ID-plug)?

What a mess... thanks in advance for your input!

It is actually pretty easy. All user selectable derates are limits during take off, but can simply be overridden by advancing the thrust levers manually. After the take off phase those user selectable derates do not apply anymore and pose no limit or problem during subsequent approaches and go-arounds. Full rated thrust is available at all times.

Why is the derate a limit? With the user selectable engine derates comes a complete set of new performance data, including Vmcg and Vmca. However during a minimum V1 take off with a derate lower than the maximum thrust level (derate 1/2) an increase of thrust on one engine during an OEI case could lead to loss of control. Very rapidly and not recoverable if not expected. At the same time lower V1s might allow one to take bigger loads out of contaminated runways, so there is a plus right there especially during winter operation.

Since derates are considered fixed thrust settings for the sake of performance calculation one can further reduce a fixed derate by combining it with an assumed temperature. To find the optimal combination of derate, ATM, flap setting and speed schedule one usually uses a computer as the possible solutions can be quite a few and ground time is limited.

Hi Denti, I had a feeling you might jump onto this ;) I guess overriding a soft derate by manually forwarding the thrust levers is obvious, but the interesting case would be a windshear encounter shortly after takeoff - when does the FMC cancel TO1/TO2 or rather what will double pushing TO/GA give you in this case?
Obviously we use performance analysis software (made originally by that big German holiday airline that had a bird on the tail once upon a time) for our takeoffs, it has a surprising talent to reduce 4000m runways to a mere 200 meters stop margin with a combination of low flap settings, improved climb and a ridiculous double derate ;)
Sure, a derate will reduce VMCG and VMCA, hence the FCTM note that during an engine failure thrust should not be increased above the green FMC N1 target unless terrain contact is imminent (which might be either the fixed derate limit or the actual double derated takeoff limit as it is on your airplanes Denti ;) )...

Pushing TO/GA twice gives you full rated go-around thrust (no derate/assumed temp).

Otherwise, the derate/assumed temp will cancel itself when climb thrust is set. If you use climb 1 or 2, it will cancel itself progressively and you will have full rated climb thrust at 15000 feet.

Derate/assume has no effect on the remainder of the flight.

Quite correct. The take off thrust is canceled same as during any normal take off when TO/GA is replaced by CLB/1/2 thrust or by double tapping the TOGA buttons.

In a one engine situation the latter is not a problem if already at V2+15 (at least in our case) of the lowest derate as that covers Vmca of the non-derated thrust limit. With both engines operating there is no problem at all of course, and to be honest in a real severe microburst like windshear i wouldn't hesitate in pushing those levers as far forward as possible.

Wrong.

First of all,if conditions of Windshear exist,Boeing does not allow to derate,so thats easy no?Your colleagues should know that.

A derate as mentioned is a LIMIT IF you have an engine failure.As your v2 selected speed takes in account the engine rating for VMCA.
If you used derate and you do get into a windshear, you are ALLOWED to increase thrust to MAX on BOTH engines.
If you chose to derate, its the same as changing your engine...your friends fly with 24 K engines also no>?well i guess they are not worried then,are they?


In a go around,you will have full thrust available.(26K or 24K depending of your engine rating(FMC) with the AT ON and MAX thrust 27.3 if you firewall them.

There has been reports of the AT not giving required thrust during a go around after an assume temp was performed.
it was due to the fact the aircraft had not reached its washout altitude.
Therefore if you use ass temp and return to land for some reason ,a go around should be with AT OFF and correct GA N1 briefed from QRH.

The EEC will monitor the fuel via the HMU to avoid N2 overspeed.


ASSUME TEMP is lying to the FMC to reduce your thrust, your perf for a go and reject are inherently safe as your calculated speeds are for the hotter temp.
if you reject,your TAS will be less as real temp is colder.
if you go ,your perf will be better (higher climb gradient) as the OAT is colder that what you set in the FMC.

If you derate and ASSume, you can get rid of the assume temp by clicking TOGA after 800 ft when the AT is not in HOLD mode anymore,or by disengaging the AT before or after 800 ft and setting the required thrust manually.

Your engine may have a plug to 26 K so firewall will not achieve 27.3k, you will have a placard in the flight deck in that case.(and i believe on the engine name plate,in the config space,the engine version).

you are correct.

I think your colleagues (captains?) should dust off their books such as the basic FCTM for a start....:E
Never too late i guess...