Incorrectly inserted Assumed Temp: Extra R'wy??

[duckbelly]

Last week we came close to taking off with an incorrectly inserted assumed temp, ie Calculated assumed temp was 50, inserted was 58.
Anyone have a rule of thumb to work out the extra runway that we would have used with the reduced thrust? (Short of calling up the Performance people.)
Not really a big deal on long strips,(long as it's not toooo much of an error) I suppose, but I would imagine it might cause an increase in interest on 2000m-ish runways, if one chose to use an assumed.
This was for a 73-800
Ta
Duckbelly

[IRRenewal]

Not really a big deal on long strips

Can't give you the number you're after, but on a long strip your are likely to be climb limited rather than field limited. The fact that the extra rwy is available to get airborne isn't going to help you if you have an engine failure at V1 and not enough thrust to climb away on the remaining engine.

On a shorter runway, the problem is not getting airborne on two engines but stopping. It will take longer to get to V1 so that by the time you (nearly) get there and you have a failure there might not be enough rwy left to stop.

Since according to your post it didn't actually happen in the end I would call up the performance people and have them run it through the Boeing software they use. Could make an interesting little piece for your company safety newsletter.

[hetfield]

I'm afraid this happens quite frequently. In my company flex temp is part of the checklist for the type I fly, but isn't for other types.

[Old Smokey]

The amount of extra runway required will all be a bit academic really, if you were field limited. You will get away with it if obstacle limited and an engine doesn't fail.

The whole background to Assumed / Flex temperature thrust reductions is to take a 'lighter than limited' aircraft to a LIMITING situation by utilising lesser thrust as though the aircraft was Field (or Obstacle) limited. Thus, you have artificially put yourself in a limiting situation, and there is no additional field available. In an RTO situation from V1, you're screwed!:eek: For an all engines departure (no failure), you'll be utilising the 15% extra margin provided for Field requirements, and you'll PROBABLY get away with it, but if an engine fails.......refer to the RTO comment.:eek:

Regards,

Old Smokey

[Guzzler]

Not an issue if you get airborne and suffer a failure as long as you select full power on the remaining engine I would of thought.

[john_tullamarine]

(a) don't folk these days still both check the performance figures ... ie, typically F/O prepares and captain runs either rule of thumb, familiarity, or even book checks ?

(b) .. as long as you select full power on the remaining engine .. be careful of dynamic/Vmca effects .. I was part of an accident investigation some years ago where pushing up the operating throttle very probably was the main cause of the crash ...

[barit1]

But - regardless of Flex temp, isn't Vmca based on the rated thrust? :confused:

[Old Smokey]

Yes, Vmc IS based upon the rated thrust, but what of aircraft which operate using several thrust ratings, e.g. TO, TO-1, TO-2.

For the TO, TO-1, TO-2 example given, there are 3 'official' Vmca/Vmcg figures, and there's no harm in advancing to the rated thrust of the day, e.g. TO-2, but NOT to full TO (Full Rated Thrust). Similar possibilities exist with Mutt's Jungle Jet on an APR OFF Takeoff, and the operating engine advanced to APR following engine failure.

For the aircraft that I fly (B777) TO-2 is a 20% De-Rate on full thrust, and if thrust is advanced to Full TO following engine failure, you're gonna start going sideways!!:eek:

Regards,

Old Smokey

[Port Strobe]

Given that there are 3 t/o thrust ratings, is the maximum derate using assumed temp method to 75% of full rated thrust (TO) or 75% of selected rating (down to 75% if TO-2) ?

Thanks

[littlejet]

Is it possible to remove just the assumed temperature part of reduction and continue the take off with de-rates only TO2 or TO1 on 777 or other aircraft type.
On the 744 the only possibility of removing the reduction on take off is with the second push of the TOGA switch but it will remove all de-rates and only after 400 ft AGL (HOLD to THR REF on the FMA) or fire-walling the thrust lever. Maybe deleting the assumed temp part on the thrust limit page on the fmc but the FCOM or my company SOP does not say anything about this procedure.

[FE Hoppy]

Your max assumed temp is either
a) perf-stop limited
b) Perf- go climb limited
c) Perf - go obstical limited
d) Perf - go all engine 115% limited
e) regulation 25% max limited
f) some other limit
If you set the wrong one you will either break the rules or break your aeroplane.
Some systems will prevent you setting more than the 25% reduction but thats not much protection.
Your selection, training and operating policies are all geared to prevent you setting the wrong temp, as well as shutting down the wrong engine or turning the wrong way or flying at the wrong height or speed in the wrong configuration etc.etc.etc.
Failure to do any of them correctly could have the ultimate consequence.
I wouldn't worry about how much more runway you may have used. I would worry about why the wrong temp was set and how to make sure it doesn't happen again.

[john_tullamarine]

isn't Vmca based on the rated thrust

Indeed .. but, in the event that the operating throttle is slammed open due stress or whatever some engines can put the pilot into an awkward situation if the TO has been predicated on speeds down near whatever the Vmca happens to be on the day .. looking at the accident I referred to earlier .. the engines had been modded to the maximum available (ie they put out lots of thrust), the pilots were thought to have had a confusing problem with one engine on TO which was associated with a significant thrust loss, almost certainly they firewalled the remaining engine .... and ended up in a fiery heap in a ditch off one side of the runway head. It is not just a matter of simplistic one-liners in emergencies .. he who deviates from the published procedures takes on a substantial, even if only potential, level of risk which might be associated with the deviation.

[barit1]

OK, Old Smokey, I concur - that is a point DEFINITELY worth making.

On the Triple7, how easy is it to quickly reset to TO-20 from a flex setting? What about other types?

[Old Smokey]

Hi barit1,

With the first press of the TOGA switches, all Assumed / Flex thrust reductions are removed. With the second press, all de-rates are removed when on the ground. In flight, one press only is required for full TO thrust, or fire-walling the thrust levers. We have no cockpit indication of the setting required for Rated or Full thrust, and rely entirely on the engine computer to deliver the thrust asked for. The only posible manual setting, is Full TO at fire-wall, and this DEFINATELY does not apply to all aircraft, in many aircraft (or the B777 with EEC OFF) the fire-wall position would exceed full thrust.

As FEHoppy has pointed out, the maximum thrust reduction allowed from Rated thrust is 25%, and having lower thrust ratings is one way around the regulations for an even further reduction, e.g. for TO-2 (B777) Rated Thrust is 80% of Maximum, and 25% reduction amounts to a mere 60% of full thrust if maximum Flex/Assumed Temp reduction is made. Easy to see how advancing from 60% to 100% following engine failure could get you into J_T's 'smoking wreckage' scenario. In a wind shear recovery scenario (no engine failure) a huge thrust excess is available.

The other advantage of large De-Rates (e.g. 20% at TO-2) is that the Vmcg/Vmca is much lower, allowing much lower V1/Vr/V2 for contaminated or slippery runways. Engine failure at V1 on an icy runway is definately NOT the time to advance assymetric thrust, in fact our training recommendation is to leave the thrust 'as is' until airborne following engine failure, and then thrust may be advanced to the Takeoff rating if desired.

CAUTION - Thrust De-Rates are 'customer selectable', and the 8% reduction I've quoted for TO-1 and 20% for TO-2 are airline specific (our GE powered B777s have 10% and 20% De-rates).

It would be interesting to hear from mutt as to his policy for APR OFF Takeoffs on the Jungle Jet. Is there any recommendation to advance to APR after engine failure, and if so under which circumstances (definately not at V1)?

Regards,

Old Smokey

[FE Hoppy]

Not sure what policy Mutt's chaps are using these days but for JAA/EASA certified E-Jets, take off with ATTCS(APR) OFF is prohibited. With it ON, firewalling the thrust lever will give you TO1 (2) or (3) RSV. So you cannot set more than the selected rated thrust. This is to prevent you busting the VMC thrust levels used in your TO calc.

[mutt]

Old_smokey,

We didnt introduce APR-OFF takeoff procedures, not sure if it is permitted under FAA certification, but we used the KISS principle. I dont actually see what the problem is with doing it as we would just produce takeoff performance based upon APR-OFF with the associated weights and V-speeds, therefore crews wouldnt have to increase thrust.

FE_Hoppy, dont forget to add "Limited to climb 1 thrust" to your list. (E170 specific)

Mutt

[FE Hoppy]

MUTT.
Depends on the FADEC load.
Some have that restriction removed now. It can be a bit odd.
TLA back from TOGA position to climb, rating changes to CLB, thrust goes up!!!

Scary!

[john_tullamarine]

.. and a similar tale from the very early days of reduced thrust takeoffs at QF .. when setting climb, the captain was a little perplexed to see the FE push the levers up a tad .... whereupon Wal changed his approach to the problem so that reduced takeoff had climb thrust as a lower limit ...

[barit1]

JT, that was a real "problem" for many operators - turning up the wick when selecting CLB from reduced TO. It upset a good many people, but it was an indicator that takeoff from a long r/w demanded less of the engine (after considering ALT & Mach) than did top of climb. :8

Conversely, those intimate with the engine performance cycle saw it an ideal way to run the engine; reduced TO served as a warmup period for the more stressful climb! :}

Good thread!

[Old Smokey]

Mutt,

Surprised that you haven't investigated the feasability of APR OFF Takeoffs. For one APR aircraft that I do work for, APR Armed wins the day for obvious reasons when Obstacle Limited, but APR Off wins hands down (by a large margin) when Field Limited, in particular Accelerate-Stop, or on Wet or contaminated runways. Lower Vmcg means lower V1, and less distance travelled at the lesser thrust at Normal (Non-APR) thrust on the remaining engine for the interval between Vef and initiation of reject actions. Definately FAA approved (at least for this type), one Alaskan short-ish airport that I just did work for showed greater weights possible at APR OFF in 85% of cases compared to APR Armed.

John_T,

Your question -"don't folk these days still both check the performance figures ... ie, typically F/O prepares and captain runs either rule of thumb, familiarity, or even book checks ?"

My answer - It's on my wish list, but it never seems to happen consistantly. Between my last post and this one, on a hot and highish airport near where mutt lives, the F/O and I independantly prepared the takeoff data. Now comes the cross-check.

F/O - "I figure we can go at TO-1 and Assumed temperature 36 degrees"
Me - "The outside temperature is 42 degrees"
F/O - "Yes, we can go with Assumed Temperature 36"
Me - "No we can't, that's more than available at 42 degrees, but we CAN go at Full TO-1 and the Improved Climb Speeds"
F/O - (Shuffle shuffle, scratch head, look at Airport Analysis from every angle), "Yes, I guess that we can, but I don't see how"
Me - "We'll talk about it in cruise, let's compare your new figures"
(Figures agreed, now comes loading the speeds into the FMC)
F/O - "There's something wrong with the FMC, the Analysis speeds are Waaayyy above FMC speeds"
Me - "They're supposed to be, let's talk about it in cruise"

We did talk about it in cruise, and I spent the remaining 6 hours wondering why I'd bothered to take 6 freaking months writing what I thought was a damned good Performance Training Manual, when I could have been fishing, or screwing, or Ppruning, or.........

Universal understanding is still on my wish list however.

barit1,

As 411A would say, "Yup, Good ole thread!" (Thought of him when I saw one of his babies at said airport, the L1011 is a beauty!):ok:

PS - Mutt, sorry for delay, documentation mentioned in recent PM being retrieved from the archives.

Rant over.

Regards,

Old Smokey