Superfluous factored take off field length?

[hawk37]

I always hear that the takeoff field length cannot be less that 115% of the all-engine takeoff distance (Far 25 in the US). Can anyone think of a situation where the runway required could ever even come close to this 115% value?

Low weights?

Hi density altitudes?

Perhaps only possible with 4 engine aircraft and never a consideration for 2 engine aircraft?

All inputs solicited, Hawk

[Mad (Flt) Scientist]

25.113 Takeoff distance and takeoff run.
(a) Takeoff distance is the greater of--
(1) The horizontal distance along the takeoff path from the start of the
takeoff to the point at which the airplane is 35 feet above the takeoff surface, determined under Sec. 25.111; or
(2) 115 percent of the horizontal distance along the takeoff path, with all
engines operating, from the start of the takeoff to the point at which the
airplane is 35 feet above the takeoff surface, as determined by a procedure
consistent with Sec. 25.111.

25.111 discusses a takeoff with an engine failure. So depending on the single engine performance, you might get the two values (113(a)1 and 113(a)2 to be close.

The intent of the 115% is not to cover a specific case (such as your examples; these are already considered in the calculation). Rather it provides a safety margin - determined empirically, based on operational experience - which assures that the actual runway length used does not exceed that calculated as required. This covers for errors in the process - both errors in flying the takeoff which might increase the distance, and also errors in weighing, loading, calculations, etc.

[oldebloke]

It's been generally regarded that the all engine gross weight takeoff case applies of 115%,as the longest distance for the heavies..
The heavy twin ,engine out case, is regarded as the 'longest'distance required !!
eg:the 747 out of 'hot and high'can take longer to use the 115%,than the same case ,engine out,to 35'
The twin takes longer to get,engineout,to 35',than both engines to 115%
Cheers:ok:

[mutt]

Easiest way to find out what Boeing aircraft is usually limited by the 115% is to check the contaminated runway corrections, if there is only a V-speed correction the aircraft is limited by 115%. If there is a V-speed and weight correction the aircraft will be usually limited by 35ft.

Mutt.

[hawk37]

Mad Scientist, when you say “depends on the single engine performance, you might get the two values…close.”, do you feel that it may be possible for the 115% value to be actually greater that the engine fail value for a twin? Reading Oldebloke’s following post, I think he is saying that it is only on the 4 engine aircraft, and not the 2 engine aircraft, that the eng fail case could in some cases be less than the 115% case.

Not trying to pin either of you down, just trying to see if anyone has run across a case of a twin being limited by the 115% case. Or if in fact it is considered universal that twins are never limited by 115% case.

As for Mutt’s post, I’m still trying to fully comprehend.

Hawk

[john_tullamarine]

Whether the AEO or OEI case is critical for TODR is going to vary amongst Types and, most importantly, boundary conditions on the day ...

Consider the basic physics - this is what MFS is getting at ...

(a) if performance is "good" today (low Hp, low OAT, low GW, headwind, etc ..) then the old girl (no prizes for guessing who flew a majority of old girls rather than shiny new toys) will get up and GO (after a manner of speaking) in the OEI case. If the AEO factored requirement is going to be critical, this is where you would expect to see that occur. Especially if the GW is low, you should quite readily expect to see AEO TODR critical for a twin.

(b) if it is going to be a dog of a takeoff today (high Hp, high OAT, etc., etc.) then the OEI performance is going to be back at WAT limits and the AEO figures will be superior .. ie OEI is limiting.

Like most real world things, generalising to say "A is more critical than B" has very limited value .... far better to have a basic understanding (not necessarily to be able to do the calcs) of what is happening (or, as dear old, and delightfully eccentric, Prof JSM was wont to ask in lectures) .... "Why is it so ?"

For a twin, in "normal" commercial operations, the OEI case will be the usual TODR limit .. but that is not a golden rule by any stretch of the imagination ...

[hawk37]

Mutt et al: reference your post

"Easiest way to find out what Boeing aircraft is usually limited by the 115% is to check the contaminated runway corrections, if there is only a V-speed correction the aircraft is limited by 115%. If there is a V-speed and weight correction the aircraft will be usually limited by 35ft."

If I understand you, then if an aircraft is dry r/w limited by 115% all engine criteria, then the V1 speed will not be a balanced field speed. And an engine failure at V1 will mean the aircraft can just stop using far25 accelerate stop criteria. However if an engine fails at V1 and a GO instead is chosen, then the screen height will be greater than 35 feet.

And then if the r/w is now wet, and the V1 is reduced but the weight and available r/w remain the same, then an engine failure at V1 will now mean far25 accelerate stop distance can still be met, however on a GO the screen height will be reduced below that when dry.

I "think" my interpretation is correct.

But what is causing me some other thoughts is for the wet runway situation, the allowed use of thrust reversers, and a reduced screen height to 15 feet on a GO is perhaps causing some invalid conclusions.

thoughts? Hawk

[Sheep Guts]

Well I was sitting in the hot sun under my wing the other day when I saw a IL76 take off in the following conditions.

44deg C
20kts Tailwind (No Joke)
4500ft available, reduced lengthfrom 7250feet heaps of clearway
Elevation 3329feet
QNH sorry for this in inches was around 29.59

Anyway pretty extraordinary conditions.

He made it by the way. He did lick his clearway though and probabaly kissed it aswell

Sheep:rolleyes: :sad:

[mutt]

In the process of introducing a new aircraft type so don’t get to drop in here that frequently!

Mr Boeing has stated that on a 4 engine aircraft the 115% all engine distance is usually the most limiting factor, so if your RWY LWT is 390,0 and the 115% limiting weight is 380,0 then your takeoff weight will be limited to 380,0.

The dry balanced field calculation will still be balanced, however as its based on 380,0, it wont use all of the runway!

For the Wet runway, the V-speed reduction is for the accelerate stop, I guess that Boeings tests have shown that the remaining runway length will always be sufficient for the accelerate go with the 15ft screen height. I don’t know if the result will still be balanced, but you will still be on the runway.

Mutt.