Engine failure during a derated takeoff

[yonygg]

Hi all,

I was trying to figure out at what point during a derated takeoff with an engine failure can we add power without loosing control.

This is how far I got before I got stuck:
Lets assume we had a V1 cut and we are airborne and climbing at V2.
That v2 was calculated for that specific derated power we are using.
At that point increasing power will increase Vmca, up to the point where VMCA would be greater than our current speed (V2) which means we would loose control of the bird.
According to that theory the first time we could increase speed is when we increase speed (i.e. @ Acceleration Height). But how much speed increase is "enough" for full thrust? and how much would be enough the next day while the A/C would have a completely different gross weight with different atmospheric conditions?

And this brings me back to my original question:
When does it become safe to add power on a derated takeoff with engine failure?


P.S.
Another problematic related scenario I just thought of is this:
What happens if the A/C is light and after takeoff we climb at V2+10 with all engines running, and at Thrust Reduction height we switched to CLB power (which in lighter gross weights is higher than the derated D-TO power), and after that increase in power we experience an engine failure? Didn't that increase in power just increased our Vmca but we still maintain the same climbing speed?


Any thoughts would be appreciated.
Thank u

[RAT 5]

Asked and answered, to some extent, in a lengthy discussion a few months ago. It is an interesting topic so have a search. It was mostly about EFATO >V1 and just airborne, not after CLB is selected. There is a spectrum of opinions and it will save you much time.

[FlightDetent]

Nice thinking about CLB invalidating Vmca. ;) But is it really the case? If using DERATE (as opposed to AST method) surely the derate gives max achievable thrust - forming the basis for Vmca et al. - and thus CLB cannot be more than that. Or can it? I never operated a derated setup, just asking...

[*Lancer*]

Changing thrust does not change minimum control speeds - they are based on full-thrust.

Since Vmcg is lower than V1 and Vmca is lower than Vr, you can select TOGA at any time after engine failure.

[Intruder]

Lancer: True for REDUCED (ATM) thrust, but NOT for Derated (Fixed Derates) thrust!

A Derated thrust setting is the baseline for VMC calculations.

[Denti]

Changing thrust does not change minimum control speeds - they are based on full-thrust.

I guess that depends on the implementation. On the aircraft i flew that had derates available, or user selectable thrust settings (737NG), the minimum control speeds are based on the chosen thrust setting, but higher thrust is still available. So yes, using thrust that would lead to loss of control at the minimum control speeds for the lower user selectable thrust setting is very possible.

Using that kind of derate has some positive sides, it allows to get higher loads out of contaminated runways and allows bigger thrust reductions than assumed temperature alone.

However, increasing thrust after an engine failure would be a very bad idea indeed if the speeds used are close to or at the minimum control speed of the derated thrust setting.

and thus CLB cannot be more than that. Or can it? It can. Well, especially so if one uses a higher climb thrust than the corresponding reduced climb thrust. But even if one uses the corresponding reduced climb thrust it can be higher than the reduced take off thrust at light weights, but usually only if one combines a derate 2 and assumed temperature.

[*Lancer*]

Ok sorry my answer only applies to flexible/assumed temperature thrust settings: TOGA anytime.

For derated thrust, Airbus state not to select asymmetric TOGA if below F speed.

[mustafagander]

It is of the greatest importance to know what you mean when referring to "derate".

Current thinking is that derate is selecting a new, lower, thrust limit for the engines and may just cover this take off or this flight or always - think airways/landing fees and whether you need the extra performance or TOW - and engine costs may be tied to derated thrust on so many occasions. Reduced thrust is the other option, maybe assumed temperature, which leaves the limit thrust unchanged and hence Vmca is for full thrust.

When using derate, all speeds are calculated using the new, lower, thrust as the baseline, hence a throttle push can raise Vmca potentially above your current speed.

Be very careful if you want to push up the thrust, remember that certified performance will be met when using derated figures, so why mess with it?

[FlightDetent]

Denti, you got me lost. By reduced do you mean "derated" or "assumed /flex" or "any combination of " ?

My question was simple: with derated thrust option, are the CLB and MCT=CON N1 limits also reduced to be equal or below max N1(derated)?

[de facto]

The easiest is standard SOP.
If you get a V1/v2 cut,and have pre selected CLB when your V2 speed was based on a derate (i.e. 22k),then when at 400 feet and engine failure is recognized (same as engine failure requiring memory items) then Disengage the AT so no unwanted movement of the thrust lever.
If your airline uses SOPS to increase to CLB thrust before flaps are up,such an SOP would avoid possible issue.
I doubt of the gain to use such sops especially if low level offs are expected.
Airmanship,good old airmanship.

[de facto]

My question was simple: with derated thrust option, are the CLB and MCT=CON N1 limits also reduced to be equal or below max N1(derated)?

Basic indeed.NO
If you use derate thrust V2,lest say (22k) and then bump it to CLB,then your thrust has gone up to 26K,so your VMCA increased,so your selected V2 may be below your new vmc a.

[Goldenrivett]

Hi de facto,
Basic indeed.NO

Please explain how you can exceed a fixed derate thrust setting by pulling the thrust levers back to Climb Power.

When Less is More | Flight Safety Foundation (http://flightsafety.org/aerosafety-world-magazine/march-2011/when-less-is-more)
"Fixed derate thrust settings are lower than the maximum flat-rated thrust setting for the engine. The CFM56-7B27, for example, has a maximum thrust rating of 27,300 lb (121 kN), with optional fixed derates at 22,000 lb (98 kN), 24,000 lb (107 kN) and 26,000 lb (116 kN). These settings are pre-programmed in the FMC and, if allowed by the operator, can be selected by the pilot when conditions permit."

[16024]

Mustafagander asked it, but it wasn't answered: Why do you want to increase thrust?
You have already arrived at a performance calculation which provides you with enough climb performance. In other words, if you have reduced, or derated, it's because you can.

[de facto]

goldenrivet.
Please explain how you can exceed a fixed derate thrust setting by pulling the thrust levers back to Climb Power.
It would not be pulled…it would be pushed forward:E
ie.vnav take off ,AT still engaged..(no ailerons used during recovery) and Thrust pre selected on ground to CLB from auto CLB2 for example.

[vilas]

Goldenrivett
Operational Liasion Meeting Fly By Wire 2004
It says:

At high thrust levels of DERATE, the TAKE OFF THRUST may fall below the CLB THRUST
An increase of thrust must be avoided when pulling the liver to the CLB detent
An automatic transition sequence to MAX CLB has been implemented for the different engine models.
There is diagram which shows that in CLB detent for 20 seconds the thrust is frozen at derate value and in next 40 seconds gradually increases to Climb.

[Goldenrivett]

vilas & de facto,

Thanks guys.

[Denti]

My question was simple: with derated thrust option, are the CLB and MCT=CON N1 limits also reduced to be equal or below max N1(derated)?

Depends on your aircraft. On the 737NG the answer however is simple, CLB and MCT are based on the full certified thrust, not the derated thrust option. Same for the reduced climb thrust settings 1 and 2, they are based on full thrust and reduced from there.

[FlightDetent]

Thanks, you quenched my thirst :)

[vilas]

Same is the case for Airbus. DERATED CLIMB applies to CLB phase so its use is completely independent of that of the DERATED TAKE OFF. Derated climb is not available for A320 variants only available for long range aircraft.

[yonygg]

Thank you guys for all the answers.

Here is the scenario:

The following data is set into the FMS:
Acceleration height : 3,000
Thrust reduction height: 800
E/O acceleration height: 1,500
D-TO-2 (the highest Derate available)
FMS recommends CLB-2 but we take CLB-1 (it saves fuel since you reach cruising alt. faster). Lufthansa once checked it and came up with that conclusion. We just follow them on that.

Assumed temp. : 50 degrees
Plane is light (~250 tons)

I don't have the numbers for D-TO-2 power or CLB-1 power but this scenario happens often and N1 (or EPR) for CLB-1 is higher than D-TO-2 power.

Performing a VNAV takeoff.

Rotating at Vr
climbing at V2+10
While crossing height 800' VNAV changes from D-TO-2 power to CLB-1 power and increases thrust (thus creating a new and higher Vmca).

So far so good since we didn't loose any engine yet.
But Murphy (or the sim TRI) now takes one engine (or 2 on the same side if he didn't drink his coffee that morning).
Now what?
Did someone check wether in that scenario the new Vmca is still higher than V2 (+10)?
And what if we now encounter a windshear and need more power, or let's take a more probable scenario - our E/O recovery wasn't perfect and we didn't meet the planned performance calculation and now our new flight path is lower than the calculated one, and we also deviated 10 degrees off course, and up ahead there are some obstacles or terrain, and the next thing we hear is the GPWS going off and we must add power. Now what?

Ok I know I got carried out with my second scenario with the GPWS but the First scenario with the higher CLB power definately happens on a daily or weekly basis.

Does the performance calculations take that scenario into consideration?


* also, in the meanwhile, I asked around and found out that Lufthansa used to take these scenarios into consideration back in the days before EFB was available and they used to check for every debated takeoff the Vmca for full thrust for that takeoff, and they used to write it on the takeoff card, and in that case they had an available reference in case they HAD to add power.
But nowadays when we use EFB that is no longer possible since the OPT (the performance calculations app) does not provide any data for Vmca...

[Goldenrivett]

Hi yonygg,
Did someone check whether in that scenario the new Vmca is still higher than V2 (+10)?
If no body checked and control became a problem then reduce the thrust (to TO Thrust). You can't be any worse off than having an engine failure at V1 and keeping TO Thrust to AA and clean wings.
our E/O recovery wasn't perfect and we didn't meet the planned performance calculation and now our new flight path is lower than the calculated one,
Since you had all engines operating until >800 ft then your flight path must be better than having an engine failure at V1.
and we also deviated 10 degrees off course, and up ahead there are some obstacles or terrain, and the next thing we hear is the GPWS going off and we must add power. Now what?
Display Terrain on your ND, weather on the other. Your enhanced SA should discourage a turn towards terrain.

[gearpins]

Boeing recommends to allow FMC to match take off thrust selection to the corresponding CLB thrust.
In our company we are most of the time on D-TO-2 with ATM which gives us CLB-2
after clean up as part of after take off procedure we select CLB. not before.
mow I know why:):)

[john_tullamarine]

Couple of thoughts ..

(a) the Vmc concern is only relevant for rather low weights as, for higher weights, Vs becomes limiting.

(b) on whatever runway RTOW, Ops Manual, QRH, etc., data you have you should be able to locate a chart of some sort which gives either specific or generic speed schedule data against weight.

From the higher weights you should see the speed schedules reducing with weight. If, at the lower weight end, you see the speed schedules flatten out, that will be where Vmc becomes limiting and you should be able to divine an approximate Vmc value, even if it is not stated explicitly.

If the weight/speed schedule continues to reduce down to the minimum weights, it follows that Vmc is below the scheduled speeds. How far ? Good question if the figures aren't listed somewhere explicitly. In this case, I would refer the matter to your ops eng folk .. who either will know the figure or can get it from the OEM.

I would opine that the more important concern, if you do elect to increase thrust at low speed on a low weight takeoff .. do it SLOWLY. I was involved in the investigation of a fatal many years ago where, almost certainly, the departure was caused by the pilot's rapidly increasing thrust on the operating engine .. both engines were high spec'd and there would have been a thrust overshoot .. which led to a very rapid Vmc departure over the runway head .. crash, burn, most died.