Wet V1
 

I've recently received a PM relating to Wet V1, which I have taken the liberty of "cutting and pasting" below. The anonymity of the originator is preserved, as good manners would require of a Private Message. As I believe that the question posed is worthy of open discussion, I have added my reply, and open the thread to greater minds than mine -

The Message :

I know and understand the principal behind why V1 is reduced, the stopping distance is increase because breaking efficiency is reduced on contaminated rwy. What I have fail to understand is the screen height reduction. Surely wet or dry VR is at the same point and your rotation is still 10 to 12 degrees NU, therefore the thrust is the same and your angle of climb is the same. So why is there a 15ft and 35ft screen height. I see the screen height more as a marker to indicate the end of the take off and the start of the climb. I know JAR states, it is the min height achieved over the rwy before the end of the clearway should an engine fail on takeoff. But why a reduced V1 should change the height.

My reply :

Your understanding of reduced V1 for a wet runway is essentially correct, so no further comment is necessary in this respect. The answer lies in the Accelerate-Go considerations.

As you've alluded to, Vr and V2 will be essentially the same, with a much larger difference between V1 and Vr than for Dry Runway operations. In the Accelerate-Go case, which is predicated upon engine failure at Vef (below V1), the aircraft must be accelerated from Vef to Vr with one engine inoperative.

Taking some sample figures of V1=150 : Vr=155 : V2=160 for DRY runway operations, the aircraft only needs to accelerate 5 knots with OEI. If, on the other hand Wet runway data is used, more typical speeds are V1=140 : Vr=155 : V2=160. The aircraft now has to accelerate 15 knots to Vr with OEI. For a 2 engined aircraft, OEI acceleration will typically be less than 10% of normal, so the distance from Vef to Vr is much longer. (Newtonian laws apply : V^2=U^2 + 2AS), but to simplify linearly, the distance from Vef to Vr will be 3 times that for the dry runway case. Thus, Vr will occur MUCH further down the runway than for the normal dry case.

At and beyond Vr, OEI performance (as you've indicated) will be the same.

So, with all other things being equal, if "Wet" V1s are used, rotation, followed by essentially the same "Vr onwards" performance will be much further down the runway, and screen height much lower because rotation was initiated much later. There are 2 solutions -

(1) Leave the 35 ft screen height in place for all operations, and suffer a huge loss in payload (and I mean HUGE), or

(2) Legislate allowing for a lower screen height (15 ft) for Wet Runway operations.

The legislative process has opted for the 2nd choice, i.e. the 15 ft 'line in the sand' Vs the 35 ft 'line in the sand'.

By accident, you've posted to someone who has suffered a legislative authority which accepted Reverse Thrust credit for the Wet Accelerate-Stop, but insisted upon retaining the 35 ft for the GO case (Option 1). Not a difficult Performance Engineering task (just put an artificial 20 ft obstacle at the Runway End, and use Wet data), but the performance penaltys were, as stated, HUGE.

One of the toughest jobs that we ask of the remaining engine/s following failure at Vef is the acceleration from Vef to Vr. If the V1 is reduced too far, it becomes a very big 'ask' of the remaining engine/s.

Awaiting better responses, which is why I've thrown this to the open forum. The question was too good to remain private!

Regards,

Old Smokey

Hi Old Smokey,

I'm afraid I can't improve on your explanation - but dare to ask if there was a typo: "In the Accelerate-Go case, which is predicated upon engine failure at Vef (below V1), the aircraft must be accelerated from Vef to Vr with one engine inoperative."

Did you mean (above V1)?

Respectfully, RRR

rudderrudderrat,

You have just attained the 2010 PPRuNe award for speed of reply!:ok::ok::ok:

To answer in a single word, NO, all Accelerate-Stop AND Accelerate-Go performance is predicated upon Vef (The speed at which engine failure occurs), rather than V1 which is the speed at which it is assumed to be recognised, and appropriate action (Stop or Go) initiated. V1 is the "practical" (and realistic) speed that pilots work with, and discussion can always be based upon considering V1 (Thank goodness!!!!).

Regards,

Old Smokey

VEF is always below V1...

(my reply was simultaneous with Old Smokey's).

.........and if I may enter the fray, I am constantly concerned by the alacrity with which folks will go for 'wet figures' when the first of the drizzle arrives - appearing to ignore the definition of 'wet' - thinking that it is all that straightforward and perhaps not realising what a big ask it is - especially if you are r/w or obstacle limited.

Maybe I'm just old fashioned having been brought up on twin turboprops whose performance OEI was 'interesting' to say the least!

OS, I don't get the point you are making about the OEI acceleration being 'less than 10% of normal'; I understand precisely where you are going in that Para, but not that specific figure. Please could you elaborate for me? Many thanks.

Cheers
mcdhu

old smokey
 

In your message in bold, you refer to a contaminated runway. Things might have changed, but rules regarding contaminated runway refer to more than 3mm of water and/or slush etc, not a wet runway. In the contaminated case, screen height of 35 ft for dry, 15 ft for wet, was reduced to zero.

On the BAC 1-11 I remember, the difference between V1 and Vr was some 40 kts, a long wait on both engines, never mind one.

Aircraft performance legislation and certification is a fascinating subject, unfortunately somewhat ruined by the over complex performance "A" examination, which put pilots off the subject for life !

PS : Is it still on the L1011 or have things ( I hope) moved on.

I'm astonished by the rapidity of the replys.

mcdhu,

I'll fly with you anywhere:ok: I too, am aghast at the number of pilots who will grab for the Wet Runway data when the first droplet of rain falls. I'm even more aghast at one (un-named) poster who routinely uses Wet data even for Dry runways - Fine for a RTO, but disastrous for Accelerage-Go. It would be survivable, but the court of enquiry would not see it that way. Such an act is patently ILLEGAL, as the Wet Runway concessions allow for REDUCED safety margins in these, and ONLY these circumstances.

My reference to acceleration being less than 10% of normal was in relation to Engine Inop Operation Vs All Engines Operation, which has a very large thrust excess, and thus acceleration.

If a 2 engined aircraft of 100 Kg mass must meet a minimum 2.4% Second Segment minimum Climb Gradient, then simple mathematics require that the aircraft, with one engine inoperative must have a EXCESS thrust of 2.4% of the aircraft mass, i.e. a mere 2.4 Kg of excess thrust. Even very small thrust increases, such as is available with APR, although low, can considerably increase this excess thrust. For OEI calculations, one engine is redundant, it has failed. If it remained running for NORMAL operations, the excess thrust for acceleration would be massive, hence my assertion that in the OEI case Acceleration will be much less than normal (Normal = All Engines), and distance to achieve a particular speed very much longer. The "less than 10%" that I quoted was a 'round' figure, and very very generous!

Regards,

Old Smokey

Thank you OS, there can be no greater accolade than that you would be prepared to sit beside me - either seat would do, I'm equally poor in both!!

The reason there are such quick replies is that I'm sure I speak for many when I say that we all follow your inputs re Perf A (and those of JT) with great interest - it is one of those subjects about which we should never stop learning and unfortunately the advent of the Airbus FOVE and other computer related systems can lead us to mere acceptance of figures without knowing exactly why they are as they are.

Must go and pack for my holiers:ok:

Cheers
mcdhu

Being a very simple man, I like to think the results of reducing V1 under any circumstance maybe the difference between the following:

-Running off the end of the runway with the engines producing little forward thrust, at low speed, or

-Running off the end of the runway with the remaining engine(s) at full thrust at high speed.

I know which situation I would rather find myself faced with.

Anything else is just too much theory for me.

Yes I have passed the UKCAA performance A examination some 35 years ago. DC 10 it was based on, IIRC.

ZERO? That's the first time I have heard that, could you please expand on which country permitted this and in what year.

I did Perf A on the Britannia :) now I know I'm getting old.

Mutt

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 :eek:



§ 121.189 Airplanes: Turbine engine powered: Takeoff limitations.

(a) No person operating a turbine engine powered airplane may take off that airplane at a weight greater than that listed in the Airplane Flight Manual for the elevation of the airport and for the ambient temperature existing at takeoff.
(b) No person operating a turbine engine powered airplane certificated after August 26, 1957, but before August 30, 1959 (SR422, 422A), may take off that airplane at a weight greater than that listed in the Airplane Flight Manual for the minimum distances required for takeoff. In the case of an airplane certificated after September 30, 1958 (SR422A, 422B), the takeoff distance may include a clearway distance but the clearway distance included may not be greater than1/2of the takeoff run.
(c) No person operating a turbine engine powered airplane certificated after August 29, 1959 (SR422B), may take off that airplane at a weight greater than that listed in the Airplane Flight Manual at which compliance with the following may be shown:
(1) The accelerate-stop distance must not exceed the length of the runway plus the length of any stopway.
(2) The takeoff distance must not exceed the length of the runway plus the length of any clearway except that the length of any clearway included must not be greater than one-half the length of the runway.
(3) The takeoff run must not be greater than the length of the runway.
(d) No person operating a turbine engine powered airplane may take off that airplane at a weight greater than that listed in the Airplane Flight Manual—
(1) In the case of an airplane certificated after August 26, 1957, but before October 1, 1958 (SR422), that allows a takeoff path that clears all obstacles either by at least (35+0.01D) feet vertically (D is the distance along the intended flight path from the end of the runway in feet), or by at least 200 feet horizontally within the airport boundaries and by at least 300 feet horizontally after passing the boundaries; or
(2) In the case of an airplane certificated after September 30, 1958 (SR 422A, 422B), that allows a net takeoff flight path that clears all obstacles either by a height of at least 35 feet vertically, or by at least 200 feet horizontally within the airport boundaries and by at least 300 feet horizontally after passing the boundaries.
(e) In determining maximum weights, minimum distances, and flight paths under paragraphs (a) through (d) of this section, correction must be made for the runway to be used, the elevation of the airport, the effective runway gradient, the ambient temperature and wind component at the time of takeoff, and, if operating limitations exist for the minimum distances required for takeoff from wet runways, the runway surface condition (dry or wet). Wet runway distances associated with grooved or porous friction course runways, if provided in the Airplane Flight Manual, may be used only for runways that are grooved or treated with a porous friction course (PFC) overlay, and that the operator determines are designed, constructed, and maintained in a manner acceptable to the Administrator.
(f) For the purposes of this section, it is assumed that the airplane is not banked before reaching a height of 50 feet, as shown by the takeoff path or net takeoff flight path data (as appropriate) in the Airplane Flight Manual, and thereafter that the maximum bank is not more than 15 degrees.
(g) For the purposes of this section the terms, takeoff distance, takeoff run, net takeoff flight path and takeoff path have the same meanings as set forth in the rules under which the airplane was certificated.





The takeoff run on a wet runway is the greater of—
(i) The horizontal distance along the takeoff path from the start of the takeoff to the point at which the airplane is 15 feet above the takeoff surface, achieved in a manner consistent with the achievement of V2 before reaching 35 feet above the takeoff surface, as determined under §25.111 for a wet runway; or
(ii) 115 percent of the horizontal distance along the takeoff path, with all engines operating, from the start of the takeoff to a point equidistant between the point at which VLOF is reached and the point at which the airplane is 35 feet above the takeoff surface, determined by a procedure consistent with §25.111.

OS
I am with you on the practicalities of not being too quick off the mark to push the "wet" button, but is it not taken care of (at least under EU-OPS and its offspring) by the requirement of 1.490 b 5:
"on a wet or contaminated runway, the take-off mass must not exceed that permitted for a take-off on a dry runway
under the same conditions."
I appreciate that this may be a geographical issue!!

Have now actually thought about it, and of course even if the mass is the same the lower V1 results in a lower screen height in the accelerate-go case.:eek:

Hi all,

Thanks for the great indepth explanation ! Very Impressive.

However, I am puzzled at the statement :-

"Wet data even for Dry runways - Fine for a RTO, but disastrous for Accelerage-Go. It would be survivable, but the court of enquiry would not see it that way. Such an act is patently ILLEGAL, as the Wet Runway concessions allow for REDUCED safety margins in these, and ONLY these circumstances."

Can somebody explain why is there a reduced SAFETY margin for wet runway ?

Thanks
lion-g

Have a good read of O-S's comprehensive explanation at #1

Hi OS,

mcdu's explanation for the speedy replies says it all.

@ lion-g. The safest way to look after an aeroplane is to leave it in the hangar permanently. Commercial operation involves some acceptance of "risk". I believe the 15 feet clearance from a "wet" runway was acceptable to the legislators because there are far fewer "wet" take offs than "dry" - hence the risk factor is lowered and acceptable.

As mcdu pointed out, choosing a "wet" V1 when it is only damp distorts the safety margins and risk factors adversely.

Hi guys,

Thanks for the prompt reply.

Do correct me if I am wrong but from my understanding, for WET calculation, the TOR for OEI is calculated based on the distance from the 15ft screen height BUT the performance calculated will take into consideration V2 to be achieved before reaching 35ft above R/W surface.

Clear as mud ?

lion-g

Hi OS, i guess you meant me by the allways wet performance data thing. Yes, we did that. However we had to still reach a screen height of 35ft and reverse thrust was calculated in. Since it was purely very short haul flying on 737s performance penalties were never an issue at all since we we never reached a weight above 8 tons below certified max weight anyway, and that was with tankering for three sectors. Data was limited to one set of performance data for ease of use and to keep the volume of data in limit, one book was enough for the whole of europe. Oh, derate was not permitted either, only ATM was used, nor did we use improved climb speeds to increase useful load or increase thrust reduction. And it was all done with approval by the relevant authority.

Nowadays we use an EFB with boeings performance tool which in standard settings always combines ATM up to the maximum with derate, improved climb speeds and aft CG options which results in nearly every take off using less than 75% of certified thrust, V1s in the mid-160s and well in the red-white lights with rotation usually deep into the red-red which took some time to get accustomed to. However we now have to distinguish between wet and dry with the normal different screen heights, which after all is only a selection in the program.

I have to say i really do like the old fashioned method, simply because the many restrictions on performance options meant we usually were in the air around mid-runway instead of now less than 300m from its end (even on a 4000m runway) if we do it by the book. I do agree however that for longrange operation where you tend to be much closer to the weight limit the performance penalties for 35ft on wet runways are a big issue, for shorthaul they are usually not.

Mutt
 

Sorry to tell you this, but it was the UK. The wording was slightly vague, along the lines of " in the contaminated situation there is no guarantee of the screen height achieved in the event of a failure.."

I think I have it somewhere in my loft, if so I will give you chapter and verse.

I too did perf "A" on the Brit, but then joined BUA on same, so felt quite at home, especially as the aircraft flight manual was much easier to use than the CAA performance "A" document. I have a feeling that we had no protection in the contaminated runway case.

Good thread. I am one who had been of the opinion that if there was some question over whether the runway was wet or not, I'd just call it wet. I now understand that this is not a good thing and that V1wet should only be used on a legitimately wet runway, though in a Dash 8 on 2200 m runways it's probably not so critical.

The real worry is not the dry/damp agonising .. even moderately wet, if grooved, is not a big deal .. the worry is the wet/contaminated consideration.

For me, if I can find a way to delay until that wall of water has drained .. that's the go. Different matter, of course, in the icy slush latitudes and a more pragmatic approach to things is necessary.