Wet V1

[Old Smokey]

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

[rudderrudderrat]

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

[Old Smokey]

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

[Kerling-Approsh KG]

VEF is always below V1...

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

[mcdhu]

.........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

[robert f jones]

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.

[Old Smokey]

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

[mcdhu]

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

[doubleu-anker]

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.

[mutt]

. In the contaminated case, screen height of 35 ft for dry, 15 ft for wet, was reduced to zero. 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

[Pugilistic Animus]

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.

[BizJetJock]

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:

[lion-g]

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

[BOAC]

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

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'.

[rudderrudderrat]

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.

[lion-g]

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

[Denti]

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.

[robert f jones]

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.

[AerocatS2A]

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.

[john_tullamarine]

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.

[Old Smokey]

Just to clear the air on a few well considered responses........

BizJetJock, Yes, I'm well aware that the JAR does require that the normal 'Dry' Takeoff Weight is the upper limit of Wet Runway considerations. You are right, it IS a geographical difference, not all countrys make that distinction in their legislation. In my Performance number crunching, I use a composite of JAR 25, FAR 25, and the Australian CAO 20.7.1B. There are various reasons for this (mainly to satisfy all of the regulatory authorities), but also as a safety issue.

In my earlier computer runs to calculate RTOWs, I encountered about 10% of cases where the Wet RTOW exceeded the Normal Dry RTOW. After much thought, I realised that 2 major safety margins are eliminated for Wet operations, thus leading to better weights with Wet figures, as Denti and PBL might say - "Daran habe ich gar nicht gedacht!". From that time onwards, I've used the 'composite legislation' approach.

lion-g asked "Can somebody explain why is there a reduced SAFETY margin for wet runway ?". Normal rules apply to the dry runway condition where (i) one means of stopping the aircraft must be held in reserve. Typically, Reverse Thrust is the reserve means not considered, and it's contribution are a bonus, and (ii) The screen height is reduced from 35 ft to 15 ft.

THE LEGISLATION OBLIGES US TO FOLLOW THE NORMAL DRY RUNWAY RULES, WITH CONCESSION AGAINST THE 2 SAFETY REQUIREMENTS MENTIONED ABOVE ONLY IN THE CASE OF WET RUNWAY OPERATIONS. To do otherwise is patently ILLEGAL.

Let's take a case where the runway is Dry, but you check Wet runway data and find that the 2 RTOW limits are exactly the same. Some cowboys will use the Wet data for an "improved" V1, but have effectively eliminated the afore-mentioned 2 safety considerations REQUIRED - ILLEGAL! If you suffer an engine failure before V1, you have obviously improved your RTO performance even without Reverse Thrust (because of the lower V1). If the failure is recognised a micro-second after V1, your screen height will be 15 ft, which is much less than the MANDATORY 35 ft required for Dry runways.

It is illegal to use a concession outside of the parameters which allow the use of the concession. Do so at your own risk, but please not when I, or my family, are sitting down the back!!!!!!!:*

Regards (but not to the cowboys),

Old Smokey

[lion-g]

Hi,

Yes Sir !! Got your points loud and clear ! Thank you for your inputs.

Just to digress a bit, on my fleet (AIRBUS), the takeoff off speeds are "fixed" for a certain weight, regardless of WET/DRY/CONTAMINATED/BRAKES DEACTIVATED !!!! The only difference will be the STOP Margin given on the RESULTS page in the LPC.

We are taking off with a very LOW V1 everytime. Even if I put in 8 brakes deactivated into the T/O Calculation, the V-speeds will still be the same!!! Only the MTOW(perf) as well as the STOP MARGIN changes.

Is this legal ?

Regards,
lion-g

[Old Smokey]

Hi lion-g,

I must admit to deliberately waiting a day or two to respond to you, in the hopes that some Airbus knowledgable person might reply, I'm strictly a BCAR, FAR 25, and CAO 20.7.1B nerd. The JAR I respect greatly, but not done any work with any of their products (apart from plagiarising a few good ideas):E).

All of that is a round-about way of saying that I do not know much about how Airbus do things.

To go to your last sentence "Is this legal ?", yes it is, but quaint, a bit 1950'ish.

Having already admitted to not knowing Airbus' mode d'emploi, my reaction is that they've over-simplified things at the expense of optimisation of Takeoff Weights. As you've said, for a given weight, the speeds are constant irrespective of the other variable conditions. What the Airbus engineering people have now done is to make the MTOW(perf) as well as the STOP MARGIN 'fit' the speeds. That's safe, but off-optimum. On the other side of the Atlantic they would have taken the variable considerations (including MEL) and optimised a Weight / Speed schedule for the condition.:ok:

Boeing are 'guilty' of doing the same thing for some MEL conditions (to keep things simple), but for more routine semi-unusual cases such as Wet Runway, optimised a whole new set of data. An example of this would be dispatch with a Hydraulic pump U/S, which directly affects gear retraction time and distance, thus imposing a 1st segment penalty. The technique used would be to use the normal V-Speeds, but apply a weight penalty to the MTOW.

A good idea to wait for a response from an Airboos expert, these remarks are just my thinking out aloud:confused:

Regards,

Old Smokey

[Max Angle]

my reaction is that they've over-simplified things at the expense of optimisation of Takeoff Weights.

That is exactly what they have done. We used fixed speeds for all runways, wet or dry, when we first had the Airbus which made life very easy at the expense of RTOW. A sheet was provided with a fixed speed table and set of flex temps vs weight for each runway we used on the scheduled network, the RTOW penalty was not a problem as we only operated short haul from fairly long runways. As the fleet increased in size we operated to more destinations, did longer flights and used some shorter runways the system was dropped in favour of standard optimised runway pages that allowed higher weights and a more optimal flex.

[Parkbremse]

Old Smokey, while i appreciate your knowledge and your experience i think you are not 100% correct on this one.

Lets suppose you are conducting a T/O from Dry Runway with Clearway, using Dry Performance Data. Regulations demand that, when factoring in the clearway, that in the accelerate - go case half of the distance from Liftoff to the point where the 35ft Screenheight is reached must be over the runway (sorry for that formulation, i'll try to provide exact references later). So overhead the Departure End of the Runway, you'll have gained 35ft/2 = 17.5ft height. However, this figure will be a tad lower as climb performence increases with increasing speed and will be almost exact 15ft.

Now lets do the Takeoff again on the very same runway, same conditions but now using Wet Performance Data. The Screenheight is reduced to 15ft, however you are not allowed to factor in the clearway (at least with B777 Performance Tools but i'm thinking this is a general requirement) so when you reach your screenheight of 15ft you'll be at the end of your Take-Off Distance Available which is the Departure End of the Runway.

Exactly the same as when using the Dry Performance Figures! And no reduction in safety.

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.

So again no reduction in safety.

The only thing which Wet Data really improves is your accelerate - stop performance. Should this result in a higher RTOW compared to a RTOW when using Dry Figures, the latter are used.

So in short, there is no reduction in safety when using Wet compared to Dry Performance Data!!


I stand to be corrected however.

[FE Hoppy]

@Parkbremse

not quite.

the difference is in the take off run which include half the distance to 35ft dry but all the distance to 15ft wet. Otherwise clearway is used the same for take off distance.


CS 25.113 Take-off distance and take-off run
(a)Take-off distance on a dry runway is the greater of –
(1) The horizontal distance along the take-off path from the start of the take-off to the point at which the aeroplane is 11 m (35 ft) above the take-off surface, determined under CS 25.111 for a dry runway; or
(2)115% of the horizontal distance along the take-off path, with all engines operating, from the start of the take-off to the point at which the aeroplane is 11 m (35 ft) above the take-off surface, as determined by a procedure consistent with CS25.111. (See AMC 25.113(a)(2), (b)(2) and (c)(2).)
(b)Take-off distance on a wet runway is the greater of –
(1)The take-off distance on a dry runway determined in accordance with sub-paragraph (a) of this paragraph; or
(2) The horizontal distance along the take-off path from the start of the take-off to the point at which the aeroplane is 4,6 m (15 ft) above the take-off surface, achieved in a manner consistent with the achievement of V2 before reaching 11 m (35 ft) above the take-off surface, determined under CS 25.111 for a wet runway. (See AMC 113(a)(2), (b)(2) and (c)(2).)
(c)If the take-off distance does not include a clearway, the take-off run is equal to the take-off distance. If the take-off distance includes a clearway –
(1)The take-off run on a dry runway is the greater of –
(i)The horizontal distance along the take-off path from the start of the take- off to a point equidistant between the point at which VLOF is reached and the point at which the aeroplane is 11 m (35 ft) above the take-off surface, as determined under CS 25.111 for a dry runway; or
(ii)115% of the horizontal distance along the take-off path, with all engines operating, from the start of the take-off to a point equidistant between the point at which VLOF is reached and the point at which the aeroplane is 11 m (35 ft) above the take-off surface, determined by a procedure consistent with CS25.111. (See AMC 25.113(a)(2), (b)(2) and (c)(2).)
(2)The take-off run on a wet runway is the greater of –
(i) The horizontal distance along the take-off path from the start of the take- off to the point at which the aeroplane is 4,6 m (15 ft) above the take-off surface, achieved in a manner consistent with the achievement of V2 before reaching 11 m (35 ft) above the take-off surface, determined under CS 25.111 for a wet runway; or
(ii)115% of the horizontal distance along the take-off path, with all engines operating, from the start of the take-off to a point equidistant between the point at which VLOF is reached and the point at which the aeroplane is 11 m (35 ft) above the take-off surface, determined by a procedure consistent with CS 25.111. (See AMC 25.113(a)(2).)

[Parkbremse]

Thanks for the reference, this what i was looking for and actually undermines my point. (But stated in clearer english i suppose...)

(c)If the take-off distance does not include a clearway, the take-off run is equal to the take-off distance. If the take-off distance includes a clearway –

(1)The take-off run on a dry runway is the greater of –

(i)The horizontal distance along the take-off path from the start of the take- off to a point equidistant between the point at which VLOF is reached and the point at which the aeroplane is 11 m (35 ft) above the take-off surface, as determined under CS 25.111 for a dry runway; or
(ii)115% of the horizontal distance along the take-off path, with all engines operating, from the start of the take-off to a point equidistant between the point at which VLOF is reached and the point at which the aeroplane is 11 m (35 ft) above the take-off surface, determined by a procedure consistent with CS25.111. (See AMC 25.113(a)(2), (b)(2) and (c)(2).)

(2)The take-off run on a wet runway is the greater of –

(i) The horizontal distance along the take-off path from the start of the take- off to the point at which the aeroplane is 4,6 m (15 ft) above the take-off surface, achieved in a manner consistent with the achievement of V2 before reaching 11 m (35 ft) above the take-off surface, determined under CS 25.111 for a wet runway; or
(ii)115% of the horizontal distance along the take-off path, with all engines operating, from the start of the take-off to a point equidistant between the point at which VLOF is reached and the point at which the aeroplane is 11 m (35 ft) above the take-off surface, determined by a procedure consistent with CS 25.111. (See AMC 25.113(a)(2).)

So:
Dry Runway: TOR = Start of T/O Run until point equidistant between v_lof and the point where 35ft is reached. At that point your height will be 15ft. (paragraph c1(i) )

Wet Runway: TOR = Start of T/O Run to the point where 15ft are reached. (paragraph c2(i) )

Exactly the same. Note that it says Take-Off Run and not Take-Off Distance The difference is that in the WET case your Take-Off is completed at 15ft screenheight so TOR = TOD which is practically the same as disregarding the clearway.

[AeroTech]

Hi,

Engine failure is taken in consideration on takeoff performance. Besides OEI acceleration, are there any other factors that affect takeoff performance and are taken in consideration for the calculation of aircraft performance?

In certain cases such snow, slush, and standing water where both acceleration and stopping are affected. How huge is the payload loss (may be a number if possible)?

Feedback appreciated.

Thank you.
Regards

[mutt]

In the case of contaminated runways, impingement drag, retardation affects, lateral control and the benefits of ploughing are taken into account. As for an average figure, there isn't really such a thing with today's computerised programs, however with our old Classics we used 10% of the runway limit weight as an average reduction.

However I do stress that under our regulations, contaminated runway data isn't certified so we legally do not have to account for anything, but if we do make corrections, we can calculate takeoff weight without accounting for an engine failure :):)

But there is a difference between legal responsibilities and moral responsibilities.

Mutt

[PantLoad]

In the case of the Southwest accident at Chicago's Midway airport, were these guys landing on a wet runway or a contaminated runway?

Mutt's last post brought this question to mind.....


Fly safe,


PantLoad

[Mad (Flt) Scientist]

according to the NTSB factual report, the runway surface condition is stated as "snow". So actual conditions seem to have been "contaminated".

edit to add the the final report makes many references to "contaminated" runway in its analysis and recommendations, but I haven't (yet) found an explicit statement "runway was contaminated" or similar, though it seems the only logical conclusion

[PantLoad]

Thank you....

Also thinking....

Weren't they using their fancy performance laptops to figure landing distance?

I guess I have to read the report....

Point is: Performance data on contaminated surfaces are not certified. You're a test pilot.


Fly safe,


PantLoad

[Jonty]

Just to add to this. The company I work for had an SOP on one of our fleets to use wet V1s for all performance calculations. This was approved by the CAA. So I don't think its illegal. However the fleet I'm referring to didn't suffer a lack of performance, even at MTOM and one engine, it still went up like a home sick angel.

[john_tullamarine]

Just talking generally, and not in any way specifically directed toward the CAA.

However, just because Bloggs works for the Regulator doesn't necessarily mean he knows much in respect to what he is talking about .... I've seen a few Regulator folks I wouldn't feed in a fit.

On the other hand some are absolutely superb in their knowledge and competence.

Just a matter of being able to distinguish between one and the other ...

[sunbird123]

What is a wet runway?.
I ask this because i know that on a dry runway the performance figures are fairly easy to calculate. On a Wet runway it is very different,a lot of things to consider.
In F1 racing as soon as drops of water appear it has an effect on performance. Lap times increase until the cars cannot stay on the track.
On another point i believe that they use statistics to help define the regulations. Most take offs are made in dry conditions,some crews may go a very long time before doing a wet take off. So the chance of an engine failure in wet conditions are very low.
Any comments?.

[JamesBon]

CONTAMINATED RUNWAY : A runway is considered to be contaminated when more than 25% of the runway surface area (whether in isolated areas or not) within the required length and width being used is covered by the following:
1. Surface water more than 3 mm (0.125 inches) deep, or by slush, or loose snow, equivalent to more than 3 mm of water.
2. Snow which has been compressed into solid mass which resists further compression and will hold together or break into lumps if picked (compacted snow), or
3. Ice, including wet ice.

DRY RUNWAY : A dry runway is one which is neither wet nor contaminated, and includes those paved runways which have been specially prepared with grooves or porous pavement and maintained to retain “effectively dry” breaking action even when moisture is present.

WET RUNWAY : A runway is considered wet when the runway surface is covered with water, or equivalent, less than specified as in contaminated runway above or when there is sufficient moisture on the runway surface to cause it to appear reflective, but without significant areas of standing water


*According to JAR-OPS 1

My note: Never flex in case of Contaminated Runway := My question...

Do you continue considering to use wet V1 if you don´t flex?

BR.

[judge.oversteer]

Great thread Smokey,many thanks, a subject to my heart and makes Prune well worth the money! Very interesting and informative responses from guys, especially my mate Mutt.

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???

Why the difference, or is it just another one of those regulatory anomalies

Can you correct my thinking, or am I just plain wrong on this?
Best regards.

JO.

[Pugilistic Animus]

But there is a difference between legal responsibilities and moral responsibilities.

Mutt:D

Old Smokey's thread really gets to the heart and soul of these matters:D

:ok::ok::ok:

[PEI_3721]

Some info sources:-
Flight operation aviation library on SmartCockpit (http://www.smartcockpit.com/flightops)
Aerodynamics & Performance – ‘Understanding Range of V1’
Aerodynamics & Performance – ‘Wet Runway (Physics - Certfication & Application)’

Takeoff / Landing on Wet, Contaminated, and Slippery Runways (http://www.scribd.com/doc/36139142/Takeoff-Landing-on-Wet-Contaminated-and-Slippery-Runways)

[Denti]

My note: Never flex in case of Contaminated Runway

True, but you can derate which in turn can increase your maximum take off weight. Or increase your margin of error. Still, the performance is not approved and advisory only.

[BOAC]

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

http://img.photobucket.com/albums/v293/boacphotos/35bfa205.jpg

[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]

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 :ok:

[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. (http://www.flickr.com/photos/47874803@N00/1966090559/)

[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.

[Type1106]

Re your post 58

Thanks John - you learn something every day!

1106

[AeroTech]

Hi,

Because of my narrow mind I thought that Vr for OEI should be higher because of the reduced acceleration. I was reading an article (Bombardier) and it was mentioned (for 3 times) that same Vr is used for AEO and OEI conditions.

It was also mentioned in the same article that Vlof-oei is lower than Vlof-aeo and the time from Vr to Vlof is similar for AEO and OEI takeoffs.
Again because of my narrow mind I thought Vlof-oei should be higher and the time from Vr to Vlof is higher for OEI takeoff because of the lower thrust.
It will be nice if someone can post an explanation.

Thank you.
Regards

[john_tullamarine]

I thought that Vr for OEI should be higher because of the reduced acceleration.

That's a fair observation and comment.

However, we only want ONE set of speeds for takeoff. Things get sufficiently busy during a critical takeoff failure without having to do a mental rescheduling of the speeds. Hence we take the more critical case (OEI) for the speed schedule and live with the speed overshoot during the routine AEO takeoff case.

It was also mentioned in the same article that

... haven't read the article so I'm doing a bit of reading between the lines here ..

Vlof-oei is lower than Vlof-aeo

.. mainly associated with the rotation rate and acceleration to achieve the appropriate incidence after a sensibly constant time delta.

and the time from Vr to Vlof is similar for AEO and OEI takeoffs.

a function of the rotation rate which generally is constant regardless of whether there is a failure or not

I thought Vlof-oei should be higher

given that the wing doesn't know too much about what the engines are doing, why should this be so ? (I suspect a slight delta but not overly significant)

and the time from Vr to Vlof is higher for OEI takeoff because of the lower thrust

(This would be so if the liftoff OEI is precisely at the same speed as for AEO) I suggest that the comment is driven more by the rotation rate (constant so a sensibly constant time delta to the lift off incidence). The reduced thrust is going to have the OEI aircraft at the lift off attitude in the same sort of time delta as AEO .. but a slightly lower speed due to the thrust delta. This ignores the probability that the AEO/OEI incidence required may be a little different due to the speed delta but I suspect that that will be a small difference.