Changing V1 on contaminated runway

[Hang-er]

I am a student in a flight school and while reading my book, discussing with my fellow students and reading on the internet (also here) I got a bit confused... here's my question

My book says the following:
In case of poor braking conditions, e.g. a wet runway, it is desirable to reduce the V1 for safety reasons, thus increasing the distance available for abandoning the take-off.

However, in a sample examn question the answers are as follows:
On a dry runway it is recommended to decrease V1 to the lowest value
On a contaminated runway it is recommended to choose the highest value for V1.
(runways are not slippy)

Now to me this seems like a contradiction. If i think about it more here's my conclusions:
On a contaminated runway it would be good to choose a low V1, so you have more stop distance in case of a stop. However the distance to VR is bigger and in case of a go this would cause problems.
If you take a high V1, then you get to VR easily but you have less runway to stop in case you stop.

So which is it? Can someone explain to me what to do, choose the highest or lowest V1 and why?

[zerozero]

Good questions.

Terrible answer: Choose the correct answer for the test. And follow the manufacturer's recommendations when operating the aircraft.

The tough part with these performance problems is that everything is always a trade off. In real life, we're constantly trading something for something else.

At this point, I'd say, without really knowing your background, that it's more important for you to be able to discern the apparent contradiction and be able to weigh the difference between the two choices--as you've done.

Good job.

Later when you're in the real airplane and faced with slippery runways and deactivated equipment and well documented performance numbers you'll do well to carefully consider all of the various factors affecting your flight.

Sorry to evade your direct question, but every aircraft, every airport and every set of conditions is different. There are lots of variables (weight, thrust, temp/press., rwy length, obstacles, installed (deactivated) equipment) that affect V1. You'll be provided with tons of information and be expected to make a quick and reasonable decision (JUDGMENT) while considering *all* of it and dismissing *none* of it.

Good luck!
:cool:

[blackmail]

hello hang-er,

it is indeed a complex matter, but forget regulations for a moment & think fysics.
your aeroplane on take off is just a big mass gathering kinetic energy according to the simple formula: kinetic energy(Ek)(joules)= mass(m)(kg) x 1/2 x velocity square. now for every speed(called V1) you choose, there will be a corresponding stopping distance, if you elect to abort, & a corresponding go distance to reach the 35feet screen height, if you elect to continue take off, assuming a critical engine fail at Vengine fail =(V1 - 1 second). furthermore , if engine fails before attainig V1, you must stop & likewise, if engine fails after V1 you must continue the take off. lower limit for V1 is vmcg & highest limit is the lower of Vr,Vmbe & Vtire limit speed. aborting at low speed(low V1), will yield a short stop distance but will need a much longer go distance, as you now accelerate on one engine. on the other hand, aborting at high speed(high V1) will require a max braking effort & a long stop distance & is considered one of the most hazardeous maneuvers in aviation. the go distance in this latter case will be much reduced as you already are at high speed close to Vr.
now, in case of wet &/or contaminated rwy's, the general rule is to reduce your max take off weight & also reduce V1 & by doing so, reducing the required stop distance & at the same time increasing the required go distance. a last word, why make a difference between wet & contaminated rwy's? firstly, occurrence of wet rwy's is more frequent than contaminated rwy's & for wet rwy's, acceleration is not a problem, only decelerating on a wet surface is. for contaminated rwy's, BOTH, accelerating due to rwy clutter & decelerating is a problem. there is much more to this, but hope this explanation helps a little bit. remember : reducing V1, reduces:rolleyes: stop distance & increases go-distance & vice versa.
kind regards,
bm

[ftrplt]

My book says the following:
In case of poor braking conditions, e.g. a wet runway, it is desirable to reduce the V1 for safety reasons, thus increasing the distance available for abandoning the take-off.
However, in a sample examn question the answers are as follows:
On a dry runway it is recommended to decrease V1 to the lowest value
On a contaminated runway it is recommended to choose the highest value for V1.
(runways are not slippy)

In your first paragraph, it is talking about poor braking conditions; i.e wet and/or slippery runways. In this scenario, the general school of thought is to minimise your exposure to a high speed abort where the braking conditions are poor - i.e it is (generally) safer to reach your GO position as quickly as you can, get airborne following engine failure and come around and land with full length available. This school of thought can also apply to dry runways as well (i.e the first option in your exam question), to avoid high speed aborts which are generally considered more dangerous than getting airborne and coming around for a full length landing.

The second option in your sample question refers to contaminated runways (that are specified as not slippery); which is an acceleration issue, not braking - in fact in an abort case the contamination actually helps with braking. So what this is saying, it's best to have the highest V1 because following an engine failure, if you GO you have to 'push' the aircraft through the muck on the runway on 1 engine up to rotate speed; whereas if you STOP, you have the muck on the runway helping you stop. In this scenario, in general it's saying its best to maximise your time available to STOP, vice GO in the scenario above.

[Dream Land]

ftrplt,
Thanks for you explanation but here is my question, in general we are talking about rain/water, a runway is considered slippery when wet, and when really wet considered contaminated, correct? :confused:

[ftrplt]

Yes you are correct; 'wet' and 'contaminated' wet is differentiated by whether water is pooling on the runway.

Therefore, wet is not considered contaminated; contaminated is snow, slush or standing water.

In a wet (and dry) scenario your are considering how well you can brake on the runway remaining following an engine failure; i.e lower your V1 as it is safer to go than it is to brake to a stop (generally!!).

In a contaminated scenario you are concerned with how well you can accelerate from V1 following an engine failure; i.e higher V1 as it is easier to brake to a stop than it is to accelerate to rotate.

(simplistic, but pertains to the original question! When you start talking about maximising takeoff weight you are opening a whole new set of considerations)

[Hang-er]

Thanks guys, this clears it up a bit. So i guess the general idea here is when the runway is contaminated, however not slippery, it will only provide extra drag, which slows your acceleration, but helps you brake. That's why you choose a higher V1.

However, now my next question, when is a runway contaminated and not slippery? Do slush and standing water only provide extra drag? I'd think it'd make the runway slippery to...

[ftrplt]

it will only provide extra drag, which slows your acceleration, but helps you brake. That's why you choose a higher V1

sort of; you are actually choosing a higher V1 to minimise the distance required (i.e time) you have to spend accelerating thru the muck on one engine, to attain rotate speed following an engine failure; knowing that in the process if you STOP, your deceleration is assisted by the contamination. (this is now getting into balanced field length and all that stuff which is a whole topic in itself).

If you think about the extreme, if the contamination is so 'thick' that you cannot accelerate on 1 engine but maintain speed, your V1 would then have to be rotate speed to even have a GO option (simplistic - you actually still need to accelerate or maintain speed during the rotation)

However, now my next question, when is a runway contaminated and not slippery? Do slush and standing water only provide extra drag? I'd think it'd make the runway slippery to...
Today 18:16

If a runway is slippery, can it be 'draggy' at the same time??


In my post #6 above; I should have said it is 'safer to GO than it is to brake to a STOP from a high speed (i.e higher V1's)

[blackmail]

hello hang-er,
no, i think you are still a bit confused. : a contaminated rwy can be slippery as well & clutter on the rwy can, but is no guarantee for reduced braking distances.
so again keep it simple & fysical(science that is) : a high speed abort= high kinetic energy, you will need a longer distance to stop than an abort at lower speed. on a wet, contaminated &/or slippery rwy, this stopping problem is aggravated & longer stop distances are required, which might exceed the available rwy distance. so, to remain safe, a weight reduction & a speed(V1) reduction is required.
regards,
bm:rolleyes:

[ftrplt]

well actually blackmail, that is not what was being asked.

The question was why you would (might) go for a lower V1 (of a range of possible values) on a wet runway (and some would say dry also), as opposed to a higher V1 (of a range of possible values) on a contaminated runway (that was specified as not slippery).

The question wasn't about weight and V1 reductions; it is only going to confuse the issue, I have been keeping it conceptual to help grasp the concept.

[safetypee]

Hang-er, perhaps some of your confusion comes from the varied terminology in common use.
A good overview of the problem, with certification definitions is given in NPA_14_2004 Operations on Contaminated Runways. (www.easa.eu.int/doc/Rulemaking/NPA/NPA_14_2004.pdf) There is also a good briefing in the UK CAA AIC (Pink 4P111/07) – registration required at http://www.ais.org.uk/aes/login.jsp . See “Runway Operations - Risks and Factors Associated with Operations on Runways Affected by Snow, Slush or Water”.

Use of the description ‘slippery’ adds confusion; it is not a certification term. However, it is used as an ATC runway maintenance description for runways which have less than the desired coefficient of friction when wet; this is advisory information for pilots. Unfortunately, Boeing and other people in the industry also use the term (without definition) for generic ‘contaminated’ runway operations.

An operational overview of some of the aspects for landing is in Managing the Threats and Errors of Approach and Landing. (www.flightsafety.org/ppt/managing_threat.ppt)

[SIDSTAR]

Hang-er,

Some good information for you in this thread. Just to point out "contaminated" is a runway covered with 3mm depth of water or the equivalent in snow, slush etc over at least 25% of the total surface. Good luck with your exams.