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Chuffer Chadley
19th Jun 2007, 18:28
Hello!

Thanks again to all contributors to my Performance in Clouds thread.

Here's another question that's doing the rounds with me and my mates.

On p190 of 'Ace the Pilot Technical Interview' (which, as has already been established, contains quite a few errors), there is a passage on 'Wet V1', pertaining to Jet transports. Being a turbo driver, this isn't my area of expertise.
The book states that 'Wet V1', a lower speed (by 10kt) than 'Dry V1', is the last moment that a t/o can be aborted on a wet rwy. Makes sense so far.

However, the book goes on to say that the 'Wet V1 is not a V1 speed because it does not imply any ability to continue the takeoff in the event of an engine failure... and may be less [than] Vmcg. Therefore, a takeoff from a wet runway may result in a risk period between [Wet V1] and normal V1.'.

This does not make sense to me- is this right? Is there really 10kt speed range in which an engine failure results in calamity in a jet?

Thanks in advance for your responses.

CC

TheGorrilla
19th Jun 2007, 18:43
Strange indeed. I always thought Vmcg was the lowest possible limit for any V1. With Vmbe being the highest.

Nor am i convinced that a wet V1 is always 10 knots less than a dry one. Having used a whole variety of wet V1/dry V1 splits before.

gatbusdriver
19th Jun 2007, 19:32
I couldn't agree more

Vmcg<V1<Vmbe

Mad (Flt) Scientist
19th Jun 2007, 21:44
The specification of 10 knots as a delta is undoubtedly aircraft-specific (almost certainly based on the author's personal experience) and is not a prescribed number). The statement that "wet V1" may not be a "true V1" speed can be true, however....

It's important to understand the specific state of the regulations in force when any given work is published.

Today's FAR Part 25 explicitly requires that takeoff performance be provided for a wet runway. (25.105 at amdt 25-92, compared to the previous revision which only considered a dry runway).

The section on selection of takeoff speeds (25.107) which contains the stipulation that V1 have a relationship with Vmcg therefore didn't apply on wet runways prior to amdt 25-92. Strictly speaking, an OEM could have provided NO wet runway data prior to that date and been perfectly in line with Part 25.

The section on accel-stop distances also didn't worry about wet runways until amdt 25-92:
Sec. 25.109 Accelerate-stop distance.
...
(b) The accelerate-stop distance on a wet runway is the greater of the following distances:
.....
(2) The accelerate-stop distance determined in accordance with paragraph (a) of this section, except that the runway is wet and the corresponding wet runway values of VEF and V1 are used.

amdt 25-42 was the previous revision to that section and contained no mention of wet distances (the corresponding para (b) is the new para (c), and so on)

Therefore, any "wet V1" derived prior to that date was, in effect, a non-certified number; it was basically advice by the OEM to the operator, with the FAA basically adopting a "three wise monkeys" approach.

So it is very possible that V1 was being scheduled without a strict regard to Vmcg in an older aircraft; it was a judgement call between the risk of a high speed abort - and going off the end of a wet runway - and the risk of an engine failure - and going off the side of a wet runway. There was an accepted degradation in safety for wet runway operations.

Mad (Flt) Scientist
19th Jun 2007, 21:54
Additionally, for those concerned more with operations than certification, Part 121 provides explicit relief from wet runway considerations for aircraft certified to older standards ....

Sec. 121.189 Airplanes: Turbine engine powered: Takeoff limitations.

(a) (WAT limited weight).
(b) (takeoff distance (one version))
(c) (takeoff distance, second version)
(d) (flight path/obstacle clearance)
(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.


So if you aren't a 25-92 or later aircraft, there's no Part121 wet requirements.

Alex Whittingham
20th Jun 2007, 09:14
I suspect this is a carry over from the old UK CAA Performance A exam which used the L1011 as a specimen aircraft. There is a facility there that calculates a max abandon speed on a very slippery or icy runway using a heavily factored ASDR and a V1wet input from another graph. This caveat appears nearly verbatim in the specimen data, CAP 385, and is making the point that, although the data is derived from a V1 wet input the resulting speed is not a V1, but a Vstop. The quote from the CAP is:

'The resulting speed is not a V1 speed for it does not imply any ability to continue the take-off following engine failure and, unlike V1, this speed may be less than Vmcg. Thus take-off on a very slippery runway may result in a 'risk period' between the maximum speed for abandoning the take-off and the normal wet V1 during which, in the event of an engine failure, the speed is too high for a successful stop, and too low for a successful continued take-off'

Is it possible that the authors have mistakenly applied this very specific caveat to all values of wet V1?

Alex Whittingham
20th Jun 2007, 16:59
But this is not a V1, it is a max abandon speed, Vstop. In the stop case the first thing you do is close the throttles. Thus the issue of directional control with asymmetric thrust disappears.

Nil further
20th Jun 2007, 18:12
The 146 contaminated runway performance works the same way as Alex has outlined (if i remember correctly) .One could calculate a Vstop and a Vgo , you then reduced your take-off weight as much as poss' to try and get the two close together .
There was a caveat in the book that stated the data was advisory in nature and had not been validated by flight test !

Try explaining that at the Board of Enquiry >

With regard to the generic 10 knot reduction , again the 146 was similar , there was a table in the performance book which gave you a number i think between 7 and 10 knots depending on runway length which gave you a Wet V1 (usual Vmcg caveat applied).

Old Smokey
21st Jun 2007, 13:01
Excellent input from the previous posts. It is indeed true for older aircraft types, that continued to soldier on using the "grandfather clause", a nominal reduction in V1 was often given to provide a Vstop, but followed by a large grey area between the lowered V1 and Vr, where performance was doubtful. It kept the older types flying by making SOME effort when wet runways became accountable. It took care of the worst case, i.e. the accelerate-stop.

I don't know (J_T would) the first aircraft that had to fully comply with "True" V1s on a wet runway, but it's a pretty safe bet that all modern aircraft still in production have to comply fully.

In establishing AFM data for an aircraft's wet runway performance, there are basically 2 ways to approach the task -

(1) Apply a nominal reduction, like 10 knots, with the caveat that V1 must not be less than Vmcg, and determine the weight reductions that must go with the requirement to accelerate an extra 10 knots from the new V1 to V2 at the end of the TODA/TODR, OR

(2) Re-engineer the whole wet takeoff package from scratch (as opposed to applying adjustments to the Dry Takeoff data) such that "True" V1s (True Stop or Go speeds) are obtained.

The author of the offending text was correct for a bygone era, but out of synch with modern day requirements.

Regards,

Old Smokey

BOAC
21st Jun 2007, 15:40
Is there really a xxxkt speed range in which an engine failure results in calamity in a jet? - certainly was in the 'old days' as stated above. Eons ago as a young steely fighter pilot jump-seater I asked a ?Britannia? (Bristol-like) Captain what happened if an engine went in that band. 'We have a discussion for a few seconds on what to do', he replied.:)

Chuffer Chadley
21st Jun 2007, 22:17
OK...

Thanks for the responses. In summary, would I be right in saying that modern aircraft can stop/go at V1 in wet weather, but a jet from the olden days might have a serious problem?

Looks like it to me.

Thanks!
CC

alrb211
22nd Jun 2007, 01:59
As far as I can recall.........

With respect to screen heights and obstacle clearance, when planning departure procedures, they run a slope of 152ft/nm starting from the 35ft screen height upward to at least 1500ft agl. Then they run another slope just 48 feet below this and asess this area for obstacles. If any obstacles penetrate this 48ft clearance area they apply an obstacle clearance departure procedure.

In the case of a reduced screen height of 15ft under wet conditions, obstacle clearance is reduced but still assured.

AL

BelArgUSA
22nd Jun 2007, 02:24
Hola CC -
xxx
I would not say that old turbojet planes had better or worse ability to stop, to compare with modern aircraft types...
xxx
Well to the contrary, if you consider the 727 as an example. that aircraft has outstanding braking performance, so much that V1 is equal to VR... we never considered a V1 reduction for a wet factor to reduce V1, actual V1 being way beyond rotation speed. And the 727 goes back to 1964... The original 727 had nose wheel brakes, later found unnecessary, and were deactivated as redundant, and to save on maintenance...
xxx
I am not knowledgeable about modern airplanes, as the 747s I fly presently were manufactured in the early 1980s, also with outstanding V1 aborted T/O performance, but certainly not as outstanding as the 727 is... In the 747-200 I fly currently, we reduce V1 in ANY case, runway dry OR wet, when the actual runway available, and its stopway if present, exceeds the runway limit weight in actual condition. That reduction is at times some 20 knots, but not less than VmcG. obviously.
xxx
Also to mention that 747 are permitted to operate with 1 or 2 sets of brakes deactivated (with a slight weight reduction) - and as usual, recall the factor of reverse thrust of at least a pair of engines (not accounted for in the FAR 25 performance certification) is still a bonus. I would say that the 747 has outstanding aborted takeoff braking performance.
xxx
Was not the case of some other types I flew. The worst were probably the DC8-50/61/71 series as far as brakes, yet the DC8-62/63/73 had much better brakes, much to be compared to the 707-300... On landing with a typical DC8-61, it was company policy not to touch brakes until the speed had decayed to 80 KIAS or less, and the reversers were worthless...
xxx
Knock on wood, I never had a catastrophic abort at speeds near V1, and I do takeoff where actual V1 out of the performance tables that must be in the last 3,000 feet of the runway. I would never attempt to abort at that point.
xxx
The concept of a reduced V1 (well in excess of 10 KIAS) was researched by TWA with their 707 fleet, and adopted, by many airlines, and was procedure for dry or wet runways, based on actual conditions.
xxx
All this is academic... only experience, surprisingly, tells a pilot where to take his hands off the thrust levers, and say to himself "it is a GO, no matter what happens".
xxx
:)
Happy contrails -

411A
22nd Jun 2007, 06:16
Ah yes, the 'ole 707-321 straight pipe powered aeroplane.
Brakes?
Ah...better be prepared to GO, as stopping was/could be a slight problem, wet or dry.
Reverse?
Just made noise, and lots of it.

Having rolled 11,600 feet on a 12,000 foot runway with these old aeroplanes a few times, stopping was the last thing on my mind...:uhoh:

L1011-500?
No problem stopping, best brakes I've ever experienced.
Superb.
A few were equipped with autobrakes...even better.
Stop that aeroplane on a dime.