Lower V1 & Obstacle Clearance

[AeroTech]

Hi,

A lower V1 is used to establish wet-runway accelerate-go distance. The FAA and the JAA believed that the use of a lower V1 and a 15-foot screen height provides a better balance of risks; the risk of a runway overrun is reduced, but the risk of striking an obstacle during initial climb is increased.

What do you think about this statement?

Feedback appreciated
Regards

[mutt]

Its true.... but remember that they are talking about NET GRADIENT.

Mutt

[Old Smokey]

Mutt speaks the truth, as always.

Of course it's true! Screen Height and Net Obstacle Clearance is reduced from 35 ft to 15 ft, that is 20 ft LESS obstacle clearance. In the INITIAL portion of the OEI climb, GROSS Vs NET is very small in terms of vertical obstacle clearance, it's not until later in the 2nd segment climb that the diverging GROSS and NET flight paths provide more 'fat'. (Your quotation refers to the initial climb, and so do I).

Wet runway data is a major concession to get you flying on a wet runway day, eliminating a LOT of safety margins, and yet there are still jerks who will use Wet runway data for Dry runway operations!!!:ugh:

Regards,

Old Smokey

[OzExpat]

I don't know about any of the other folks who've ever been at the sharp end of the INITIAL portion of the OEI climb, but seems to me that you have to fly VERY precisely in an emergency situation to ensure obstacle clearance of just 15 feet! That's not the sort of thing that you want to do routinely - or maybe you should? :eek:

Think about your seat height above the bottom of the fuselage and hope that the wheels aren't dangling for too long...

[Old Smokey]

To put a number on my earlier post, with a 0.8% delta between Gross and Net, it is necessary to fly 2500 ft / 762 M horizontally before the 0.8% 'fat' has 'recovered' the 20 ft lost in obstacle clearance. All of this presumes, of course, that you fly the aircraft as well as the test pilot, and achieve the full Gross performance.

2500 ft / 762 M is probably about half of the 1st segment for most 2 engined aircraft, thus, the remarks originally quoted do indeed refer to close-in obstacles.

Regards,

Old Smokey

[EMIT]

Ref Old Smokey:

... and yet there are still jerks who will use Wet runway data for Dry runway operations! ....

Whether that is jerky depends on what part they use of the Wet data: if they only use the associated assumed temperature, but not the V1 reduction, than they will have given themselves more margin.

The converse would be true of people who would use the dry data because the runway is not yet "really" wet.

[mutt]

than they will have given themselves more margin..... and find themselves in a court of law justifying why they attempted to takeoff illegally.. You cant mix DRY and WET data.

Mutt

[EMIT]

I hope that the judge will first attend a course on aircraft performance!

A performance calculation gives you the maximum mass that you can lift off the runway at prevailing conditions.

If your actual mass is less than that maximum, you can use your margin to take-off with less thrust, to lessen engine wear. You can eat up all your margin, or nothing at all. How much margin you eat is company policy, not a matter of the law. (Meaning, you are free to choose any assumed temperature between full thrust and the maximum temp that you have obtained from the performance table).

The above statement is valid for dry and wet runways!

If the runway is wet, your lift capability is disappointingly low. The lawmakers have decided that is is allowed to improve lifting performance by allowing a lesser obstacle margin (for the engine failure case). Notice that, in wet conditions, it is allowed to take off at reduced thrust.( Some could argue that that is in contradiction with the lowered obstacle clearance).

The performance problem with a wet runway is not in the acceleration part of the takeoff, it is in the deceleration in case of a stop. The V1 has to be low to enable a stop on the remaining runway. From that early V1, acceleration to takeoff speed takes longer than from the (higher) dry V1 speed, so the stop distance and the go distance will be approximately equal again (balanced).

(A takeoff does not need to be balanced because it looks nice, but because, if it not balanced, one scenario (STOP or GO) gives excess performance, while the other scenario is very limitng. Balancing both scenarios gives approximately equal masses for both scenarios, so it gives the best overall performance - remember, you are alwas limited by the most limiting factor. Besides stop and go, also climb performance is evaluated, and could be the more limiting factor on hot and high airports with a long runway).

Using wet figures usually necessitates a lower maximum of assumed temp for a given takeoff mass (that is the flip side of a lower maximum T/O mass at maximum thrust).
A lower max assumed temp means that the thrust to be used is higher. Now, if you use WET figures on a DRY runway, at the max assumed temp that is allowed by the wet table, than you will use more thrust than you would need on the dry runway - you have more margin than required.
THE ONLY THING THAT YOU MUST NOT DO, IS TO USE THE LOWERED V1 OF THE WET PERFORMANCE, BECAUSE IN CASE OF THE DRY RUNWAY YOU MUST MEET THE HIGHER OBSTACLE MARGIN REQUIREMENTS.

End of lesson.

Any questions?

[Bergerie1]

Operating off contaminated runways is one of the most difficult areas that a commercial pilot has to face. Ice, snow, slush and standing water all degrade take-off performance to varying degrees and require reductions in allowable weights. In line operations it is often difficult to obtain exact information on the depth, density and extent of the contamination, and it is equally difficult to measure coefficients of friction. Therefore the Flight Manual adjustments for these conditions are conservative. Snow, slush and standing water affect both acceleration and stopping. Ice and wet surfaces affect only the stopping.

Take-off performance is based on (a) the take-off run – all engines operating; (b) the take-off run with one engine failed at V1; (c) the take-off distance – all engines operating; (d) the take-off distance with one engine failed at V1; and (e) the emergency distance – the distance required to accelerate on all engines to V1 and then to stop. On most take-offs none of these are limiting and even when operating at max weight on a limiting runway usually only one of these is limiting. Therefore it is normal for there to be a range of allowable V1 speeds, although the performance data available to the pilot will only show one V1 in order to simplify the task.

When considering take-off performance, one is balancing the risks involved in stopping versus the risks of continuing the take-off after an engine has failed. On the large 4 engine aircraft that I used to fly (VC10, B707 and B747) it was always considered that a rejected take-off at max weight from V1 on anything approaching a limiting runway was a major emergency in its own right. And that was in addition to whatever had caused the pilot to stop in the first place. First, the pilot is faced with a difficult and unexpected decision (even though he/she should be primed for it); second, really prompt and ‘aggressive’ action (including immediate maximum braking) is needed; third, wet runway braking action is always slightly uncertain; and finally, after stopping, the brakes and tyres will be very hot and may even catch fire. Each of the first three issues can lead to an overrun off the end of the runway, and the fourth may need evacuation of all the passengers.

Bearing in mind that usually there is a range of possible V1s (even though the pilot uses only one) discussion took place back in the 1960s to see whether it would be sensible to use a lower V1 on wet runways. This was led by an excellent performance engineer in BOAC who argued that a small reduction of V1 speed in wet conditions would (a) allow more runway distance for stopping and (b) reduce the kinetic energy to be dissipated (this being related to the square of the speed). This idea was eventually adopted by the UK and later by the FAA and JAA.

Of course the trade-off was to have a lower screen height (35ft reduced to 15ft) but it was agreed that this was a small sacrifice to pay and would only occur on a few occasions as there a number of safety factors that give a ‘bonus’ to the pilot. Firstly, the reduced V1 will usually be within the range of normal V1 speeds; secondly, the aircraft will usually exceed the ‘book figures’ and will therefore exceed the required minimum of 15ft; and thirdly, take-off performance is predicated on a loss of thrust at V1, the chances of this happening in the few critical seconds approaching V1 are very small. Thus, the likelihood of passing the screen at less than 35ft, even when using a reduced V1, is very small indeed.

In addition to all of the above, most pilots also felt that, provided the aircraft was controllable, it was best to get airborne and take the problem into the air. Even in the case of an engine fire, and especially on podded engines, it was considered to be more easily dealt with in the air than when stationary on the runway.

Even though a 15ft screen height seems very little clearance, missing an obstacle by only 5ft is preferable to going off the end of the runway when trying to stop – there are too many runways with unnecessary obstacles, ditches etc. that can cause major damage.

[mutt]

On the large 4 engine aircraft that I used to fly (VC10, B707 and B747)This was led by an excellent performance engineer in BOAC who argued that a small reduction of V1 speed in wet conditions I never cease to be amazed by the wealth of talent and knowledge withing the Pprune community...... Welcome good sir.....:ok:

Mutt