Wet V1

[BOAC]

Aah! So that's what they mean by a reduced screen height..........................

[john_tullamarine]

Long lenses always help the drama.

Similar situation with AN's 727LRs at ADL with clearway credits heading off out over the bay. On one memorable occasion, years ago, there were folks working on the runway head. The crew asked through the tower whether they might like to vacate for the departure. Advised "no, thanks, she'll be right" .. the aircraft launched.

Apparently said folks dived for cover in all directions.

[galaxy flyer]

JT

Point taken on telephoto lenses, but a serious question. Somewhere here it was noted that four-engine planes are more typically limited by the requirement of 115% of AEO take-off run. Why is that while the two-engine kites are OEI limited?

GF

[john_tullamarine]

Don't have any manuals to hand today so I'll have to generalise.

The main difference comes down to the initial climb performance.

For the two-motor-machine, one loses around 50 percent thrust with a modest increase in drag ... resulting in something in the region of 80-90 percent climb gradient performance loss comparing AEO > OEI.

As a consequence the AEO takeoff case is very sporty and the raw AEO distance (at, say, 10-15 percent gradient) generally is considerably less than the OEI (at around 1.6 percent). Even with the 15 percent penalty, AEO TODR tends to be less than OEI.

For the four-motor-machine, one loses only 25 percent of thrust, again with a modest drag increase. The magnitude of the delta between AEO and OEI is very much smaller. With the 15 percent penalty AEO often comes out the loser and AEO TODR is limiting.

As with all such stories, generalisations only go so far and one should cite a specific AFM to come up with quantitative data.


... and, with the Eva Air shot, without any knowledge of the runway, it is not at all necessarily evident that the runway head is where one might presume it to be from the photograph .... ?

Nonetheless, it is a ripper picture ..

[ShinjukuHustler]

Great topic

Thanks to Old Smokey and all who posted replies, it was a very enjoyable and educational topic. It's a pity there aren't more like it.

I did my perf A on the L1011 and it still gives me chills thinking about it I too hope it is less frightening these days.

Hustle On

[rudderrudderrat]

Hi JT,

The photographer of the Eva Air shot explains : "I put the photographers-place on the flick-map. And a little secret revealed: the horizontal distance between the fence and the 747-wing is 145 meters." here.

[galaxy flyer]

Thanks, j_T, just the answer I was looking for.

GF

[mutt]

Somewhere here it was noted that four-engine planes are more typically limited by the requirement of 115% of AEO take-off run

Oh ye of little faith.......

Mutt

[john_tullamarine]

the horizontal distance between the fence and the 747-wing is 145 meters

For those who don't play much with photography - diifferent length lenses can produce characteristic effects - in this case, the longer telephotos will cause an apparent "bunching up" of things giving the impression that things are rather closer than they really are.

just the answer

exacerbated by increasing the V1/VR split as well - OEI, the V1 to VR acceleration is reduced, giving a longer OEI acceleration distance. However, most of the difference is in the initial climb capability.

Oh ye of little faith

Numbers of motors is OK .. but, if it has fewer than four hosties, it's a light aircraft ...

[donstim]

Now suppose you perform a Take-Off on a runway without clearway eg close-in obstacles. Now using Wet compared to Dry leaves you with less height in the same spot (15ft compared to 35ft) however regulations demand that you cross any obstacle by at least 35ft and make no difference between a Wet and a Dry Runway. That means, that when using Wet Data you still have to achieve 35ft over the first obstacle and thus your climb gradient must be better. This fact is being accounted for in the performance calculation and results in more excess thrust when compared to Dry Runway Data.

This is where you're a bit off. The regulations demand that the net takeoff flight path clear obstacles by 35 feet. The takeoff flight path and net takeoff flight path begin at a point 35 feet above the takeoff surface at the end of the takeoff distance, regardless of whether the takeoff was conducted under the dry or wet rules. When the runway is dry, the airplane must be at a height of 35 feet at the end of the takeoff distance. But, when the runway is wet, the airplane need only be 15 feet above the takeoff surface at the end of the takeoff distance. Therefore, although the net flight path obstacle clearance is the same for both wet and dry takeoffs, for the wet case, the actual clearance will be 20 feet less.

The way this was done in the rules was to state in FAR/EASA CS 25.115(a), "the takeoff flight path shall be considered to begin 35 feet above the takeoff surface at the end of the takeoff distance..." That way, there was no need to change the operating rules to accomodate a 15-foot net flight path obstacle clearance for the wet case, and no need to build a new set of 15-foot obstacle clearance charts.

[donstim]

I may be missing some point that you are trying to make PA, but the FAR/EASA certification and operating requirements for dry and wet runway takeoffs are exactly the same (with the caveat that the FAR operating requirements don't explicitly state that the wet runway takeoff weight cannot be greater than the dry runway takeoff weight.

having a close look at Part 25 the emboldened part would seem to be very worrisome, and IIRC wrt to certification there are four or five scenarios for determining wet TO parameters, I wish I could find the original link, but it was an excellent presentation

I wonder the major operational difference in comparing FAR 121 with JAR ops... that perf A stuff seems a lot more comprehensive,...FAR 121 operational requirements do seem very sketchy..and not really much is said...and they seem quite marginal now after seeing how it's done elsewhere
121-189 does not look very promising at all, especially in conjunction with obstacle clearance, the screen height seems arbitrary as any obstacle must be cleared by 35' and V2 must be achieved by 35' feet..so for an 35' obstacle after the clearway the aircraft only has to achieve 70' the 'screen height' and minimum obstacle clearance...I've always looked at that one and said that can't be right-even while looking at the diagram

so, below seems to indicate that on a wet runway your net height above an theoretical 35' obstacle is only 55'along with that excerpt below from FAR 25, regarding V2

[donstim]

At this point, there are no FAA regulatory requirements for the contaminated runway case. For the wet case, both FAA and JAA (and EASA) allow credit for the use of reverse thrust. The JAA (EASA) also allow credit for the use of reverse thrust in determining contaminated runway accelerate-stop distances.

Just a quick question if I may, can you please confirm that in the wet/contaminated accelerate-stop case, the FAA figures are predicated on the use of reverse thrust, whilst in the JAA rules use of reverse thrust is not taken into consideration???

[Pugilistic Animus]

donstim, basically I was saying that it can get real tight

nothing about weight as I'm not an expert on those matters...just two things really

that in FAR 25 the screen height is lowered although the height for achievement of V2 remains the same and your actual distance above an obstacle could be dramatically reduced from an already 'lowish' figure

[AeroTech]

Hi,

exacerbated by increasing the V1/VR split as well - OEI, the V1 to VR acceleration is reduced, giving a longer OEI acceleration distance. However, most of the difference is in the initial climb capability.

JT, I don’t understand why the acceleration is reduced only from V1 (or Vef) to Vr, why not beyond Vr and up V2 (35 ft) since one engine is still inoperative?

Why Vr is the same for AEO and OEI conditions?

Feedback appreciated
Regards

[rudderrudderrat]

Hi Aero Tech,

Why Vr is the same for AEO and OEI conditions?

I thought it was because with OEI you'll accelerate towards V2, but with AEO you'll accelerate to V2+10 as a minimum.

[john_tullamarine]

why the acceleration is reduced only from V1 (or Vef) to Vr, why not beyond Vr and up V2 (35 ft) since one engine is still inoperative?

Perhaps I wasn't being terribly clear in my previous post - I was talking about the effect of V1/VR. You are quite correct that post VR the OEI case is still more critical than the AEO.

Why Vr is the same for AEO and OEI conditions?

We need to have the one VR (for simple minded pilots like me) so it is predicated on the OEI case and generally chosen so that, at the prescribed rotation rate, the aircraft achieves V2 at screen. As RRR observes, this generally gives a modest V2 overspeed for the AEO case .. unless you want an uncomfortably high body angle, especially for twins.

[Type1106]

John,

I'm a simple minded pilot too, but I understood that VR is determined by the aerodynamics of the aircraft and is wholly dependent on aircraft weight, regardless of AEO or OEI. Clearly, the wings don't know how many engines are operating just how fast the aircraft is travelling.....where it occurs on the runway is a different matter, of course.

1106

[john_tullamarine]

While the certification standards have the usual boundary conditions, generally the final choice for VR is to select a speed which makes V2 work and that is driven by the OEI case.

For other than very low weights, V2 and, hence, VR, would be expected to be driven largely by RTOW.

[FullWings]

I'm glad someone has pointed out that it isn't a good idea (or legal) to use wet figures on a dry runway. If find myself having this discussion fairly regularly at work, usually because of the misapprehension that going 'wet' is safer whereas the opposite may be the case.

There are some combinations of aircraft type, weight and runway length that give a higher TOW with wet over dry... I think some operators used to 'cheat' a bit on occasions by using wet figures inappropriately.

[Denti]

As was pointed out in this thread at least european regulation does not allow that anymore, wet TOW cannot be higher than the relevant dry one.

Anyway, Vr and V2 are figures you can play with to achieve a certain goal. Not only are they configuration dependent (737: flaps 1 through 25), but you can shift them between minimum speed figures into improved climb ones which can change speeds by up to 40 kts.