Why does V2 normally occure and 35ft above the end fence but changes to 15ft in the wet?

Question

Does it or am I confused?

Answer 1

The screen height, essentially the clearance height above any obstacle, is indeed 35 ft for dry runways and 15 ft for wet ones.

Why is it lower for wet? Perhaps it should be put the other way; why is it higher for dry?

Clear an obstacle with 15 ft, and you are clear. Clear it by 35 feet and you are just more clear; so why the difference? Safety.

If one takes off from a wet runway, the performance--accelerative and decelerative forces, most notably the impingement drag of water on the wheels-- is already factored in as a worst case scenario. The runway is wet, it can't be worst than that.

But take off from an officially dry runway, and you do not know what to expect; there may be standing water somewhere down that no one is aware of, it may start raining just as the airplane start to roll to get to the end of the airstrip; so let's add a 20 ft extra clearance for safety sake, and no one gets hurt.

Remember that the 3 most important things in aviation are safety, safety and safety (in that order).

Answer 2

You're confused. The 35 feet never changes wet, dry, snow, ice, whatever. When I give a class on takeoff performance in a Part 25 transport category aircraft, it takes a whole day to cover it all but here's the deal. The takeoff run begins at break release and continues to V1. At precisely V1, if you were to loose an engine the point where you would come to a stop on the runway if you chose to reject the takeoff would be exactly the same point you would reach 35 feet if you chose to continue. The total distance from break release to that final point is called Balanced Feild Length. But the 35 feet never changes. You may be thinking of adding 15% to landing distance on a wet runway for comercial operations. That's the only place I can think the number 15 being used in regards to wet or dry.



Caretake, my friend, I have no clue what you're trying to say here and I'm not sure you do either. Jeez man.

Answer 3

I was going to give you the exact answer that John just gave you, but then I noticed FAR 25.113... I'm guessing that's what you are asking about.



That's more for manufacturer performance certification than any operational considerations. My interpretation is that rotation is always consistent with reaching V2 at 35 feet, but one factor for determining takeoff distance is reaching 35 feet from a dry runway or 15 feet from a wet runway... There are other factors, though, and the regulation states that the longest distance of any of those factors is what's used to determine the takeoff distance.



Again, this is only guidance used for the manufacturer to develop performance charts. I have never heard of any operational implications of 15 feet...



EDIT: John, I'm type rated in a couple different jets. I didn't have access to any AFMs when I wrote the original answer. According to one AFM, it defines 1st segment climb as the segment beginning at Vlof and the gear either down or in transit. V2 must be reached by 35 feet (reference zero), which is the end of TOFL and the beginning of the takeoff flight path... The second segment climb starts at the point that the gear is up and locked.



I'm not typed in a Gulfstream, so I'm not going to call out your definition as wrong, but Is it possible that manufacturers define the segments differently? FAR 25.111 only states that:



"(d) The takeoff path must be determined by a continuous demonstrated takeoff or by synthesis from segments. If the takeoff path is determined by the segmental method—

(1) The segments must be clearly defined and must be related to the distinct changes in the configuration, power or thrust, and speed;" and so on...



It sounds like manufacturers might be allowed slight variations in the way that segments are defined.



In any case, I passed two type rides and had the takeoff profiles nailed... I had to, like you said... But that does reinforce the point that I've never seen or heard of accelerating to V2 by 15 feet instead of 35 feet. I'm assuming that the 15 foot figure is one limitation used to develop performance charts, but operationally, that's a transparent requirement because it has nothing to do with how we operate. I haven't ever heard of that until I saw it in 25.113. Let me know if I'm on the wrong track... I'm willing to learn from this. This conversation got interesting.

Answer 4

I'm going to go with Confused, in Wet conditions generally V1 will change because Braking action is assumed to be worse, but VR and V2 stay the same and V2 should be hit 35 feet above the runway......nothing to do with the Fence surrounding the airport

Answer 5

You really should check this site. This is the section on wet or dry surface, in affecting V2.



"Airfield surface and surrounds. A short dry grass or rough gravel surface might add 10% to the ground roll compared to that for a smooth sealed surface. Wet or long grass might add 50% to the ground roll and a soft or waterlogged surface might double the ground roll. Surface water and/or wet grass can lead to aquaplaning and loss of directional control, the effect of frost is similar."

http://www.auf.asn.au/groundschool/umodule11.html



In reading this again to find your answer, and the above quote is the only reference, I realized, V2 is the speed required to break ground effect. An underpowered, overloaded, aircraft could get off the ground but not be able to break ground effect.



That's why on a hot, humid day, on a short runway with obstacles ahead, you break ground, drop the nose, build airspeed till said obstacles are becoming a significant of your windscreen, then cuddle the yoke, and climb for old glory until you only have friendly forward vision.



That's a 'get out'a Dodge' takeoff. Not recommended for general aviation.



OOPS: I looked again, using find 'wet' and found this,

Obviously a take-off into wind is highly desirable, unless runway slope and rising terrain dictate otherwise, and the ground roll should be started as close to the boundary fence as reasonably possible. The procedure described above is for a hard, dry surface or for short, dry grass. If the surface is soft or the grass is long and wet then the rolling friction may exceed the induced drag at medium aoa or the slippery surface may make directional control difficult. In such cases it may be better to get the wheels off early and fly in ground effect until Vx is attained, as in the soft field technique. If there are any doubts about the take-off conditions then stay on the ground. I suggest you read the article 'Tree's a crowd' in the Flight Safety Australia September – October 2002 issue.



So that substantiates my 'get out'a Dodge' proposal



John B I'm trying to say two things.

Terminology has changed in the last thirty years I've been out of the cockpit and....



I don't know how you pro's can get it so screwed up.

V1, which we called Vr, velocity refusal, is the point and speed at which if something goes wrong you are committed to take off. i.e. Below V1, the takeoff is aborted; above V1 the pilot continues the takeoff and returns for landing. I have confirmed that in FAA regs and several other references in order to understand today's terminology, Geeesh!!!!



And I have also included source for his reference.