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Airmann
9th Feb 2017, 09:42
What's the deal with different screen heights for wet and dry runway conditions?

I thought screen heights was primarily used as the starting point for building departures. Why allow for a lower screen heights if the runway is wet? The obstacles along the takeoff path do not change with changing runway conditions

Piltdown Man
9th Feb 2017, 09:58
I think the answer is buried in the sands of time. When Perf. A as is it is called in this part of the world was introduced, four pistoned engined aircraft were the order of the day and they had a great deal of trouble flying safely with an economic payload following an engine failure. They needed more runway both to stop and fly. Lowering the screen height gave them the margin they needed. This fudge has stayed with us today and the errors associated. I can remember being able to lift more weight from some wet runways than dry ones - clearly a ridiculous situation.

So simply put, a kluge from history!

RAT 5
9th Feb 2017, 10:42
I flew for various airlines in the days of RTOW tables. One always used wet performance in the belief it gave more buffer in the stop case. Another insisted you did both calculations and took the worse answer. So on a dry runway you also did a wet calc' and on a wet runway you did a dry one. If the dry was lower than the wet, on a wet runway, you used that.
Now with i-pads I've no idea what they do, other than plug the numbers in and accept what it spits out.
Different screen heights does seem to be a fudge for commercial reasons. Apparently it has not been addressed in decades. It would be an interesting answer to hear if anyone cares to ask the EASA & FAA guru's.
Fudge for commercial reasons: now where have we heard that before?

Old practices buried in the sands of time: now where do we see that every 6 months?

thetimesreader84
9th Feb 2017, 11:07
In my JAR Perf-A groundschool years ago, the instructor, an ex DC-8, DC-10 loadmaster, flight engineer, general dogsbody who'd spent the vast majority of his 10,000 hours flying around Africa claimed it was because 35' was all the original 707 could achieve off a wet runway, engine out, and with the FAA (or CAB I guess) back then being in the pocket of Big Boeing, they bent the rules to suit.

Good story, if nothing else.

Goldenrivett
9th Feb 2017, 11:38
Some really good opinions from Old Smokey, Oldebloke, John T. etc. circa 2006. dry/ wet screen height [Archive] - PPRuNe Forums (http://www.pprune.org/archive/index.php/t-207283.html)

Airmann
9th Feb 2017, 15:37
OK the above link gives the answer pretty clearly. Clearly shows how safety is put second to profit when the regulator's believe they can get away with it. I mean why not just reduce screen height to 15' in all conditions. Why accept it when the runway is wet and accept the reduced margin in climb but still insist on 35 on a dry runway. So illogical and kind of sad.

fantom
9th Feb 2017, 15:40
Yes. It is because the stopping distance in the wet is longer than in the dry. V1, therefore, is lower. Lower screen height therefore reqd..

Airmann
9th Feb 2017, 15:49
Ya but of it's safe for wet should also be safe for dry. Like I said just use the lower of the two for both.

This kind of compromise makes me lose respect for aviation. I accept the commercial considerations but then just chose one and run with it. 15' for both. Imagine an accidental because of a wet runway and lower screen height. Imagine answering grieving families of victims and saying, well if the runway was dry....Bad luck for the rain.

galaxy flyer
9th Feb 2017, 18:01
Airmann,

I'd suggest reviewing overall accident statistics and the number of accidents due to engine failure at Vef on a wet runway at limiting weights. Engine failure is a vanishingly small problem compared to many others in aviation.

We spend hours and hours in the sim over a career on engine failures on the runway and precious little time is spent working real problems that require decision making and CRM skills.

aterpster
9th Feb 2017, 19:13
I believe both the wet and dry screen heights were 35 feet until well after the airline fleet was mostly jets. Like 1970s or early 1980s as I recall. The justification for reducing the screen height to 15 feet for wet runways was to lower the decision speed, so in the unlikely event of an engine failure during takeoff roll the odds of being able to continue the takeoff increased, which would normally be safer than trying to stop on a contaminated runway.

john_tullamarine
9th Feb 2017, 22:50
I believe both the wet and dry screen heights were 35 feet ..

Aterpster has the story ...

Originally all were 50ft ... which derived, ultimately, from a very old demo of a Curtiss Jenny (as I recall) into a military parade ground surrounded by trees of around that height. The original US regulator, having to make up some "rules" at the start of the game, figured that was a fair starting point. At least, at an FT course I did many years ago, that was the story told by an ancient FAA certification engineer who was but an office boy at the time.

Part of his discussions indicated that quite a few early rules were little more than finger-in-the-wind best educated guesses by the then technocrats. Some of these persist through until the present day. For example, the maximum stall speed for singles ... originally based on motor vehicle crash damage considerations was a best guess consistent with then aerospace reality ... they came up with 70 mph which, now, is 61 kt.

Later, heavies had the 35ft introduced, regardless of conditions. Subsequently, the sensible reality of balancing stop and go for wet conditions saw the concession to 15ft screen. Much of the history and logic of performance requirements development during the piston to jet transition derives from an ICAO report by the Standing Committee on Performance (early 50s).

It is not a simple case of the regulatory process bending to the will of commercial pressure. All certification, ultimately, comes down to the best rational assessment of risk and balancing risk against what the state of technology can reasonably achieve at the time. This, of course, is what drives periodic tightening of design rule provisions.

Jwscud
10th Feb 2017, 11:13
This is a source of much frustration. The link posted above was fascinating as to the derivation of the rules as indeed is JT's post, thanks.

Our company mandates use of wet figures when the runway is reported damp. This is not necessarily unreasonable until one sees ATIS reports of damp on an almost bone dry runway. When on a fat heavy approaching TOPL, the end of the runway seems pretty close at the best of times and the reduction in margin of wet figures is enough to leave one uncomfortable.

V1 cuts may be pretty rare, but they do happen, and are more likely with the aircraft approaching limiting weights due to the need to take off with full thrust selected.

Galaxy flyer - we spend a lot of time in the sim practicing LOFT/LOE type scenarios and I agree that sort of training is very important. However, one generally has the advantage of speed, time and altitude in such situations. The immediacy of the threat and the lack of any of those three luxuries on takeoff is what keeps me thinking about it.

john_tullamarine
10th Feb 2017, 12:26
is enough to leave one uncomfortable.

.. but are you not taking some benefit of a V1 reduction ?

I'd rather be faced with a reject on a dampish runway from a lower V1 ... unless there be significant very close in obstacles, reducing the screen height wouldn't cause me to lose much sleep having traded the associated reduced gross/net margin for an improved reject capability.

This FAA video (https://www.faa.gov/tv/?mediaId=1133) may provide some useful thoughts for the discussion

Airmann
10th Feb 2017, 13:40
Airmann,

I'd suggest reviewing overall accident statistics and the number of accidents due to engine failure at Vef on a wet runway at limiting weights. Engine failure is a vanishingly small problem compared to many others in aviation.



Galaxy Flyer, I'm not really arguing that 15' isn't safe. Its more that I'm a little bit concerned with the fact that the two numbers are different. I mean why? Just chose the lower one and stick with it. I mean why should they keep the Dry screen at at 35' when a shower of rain which stops before I take off suddenly changes my screen height. I completely understand why its done, but I mean I find it strange that they haven't also lowered the dry screen height to 15' and just left it at that.

We spend hours and hours in the sim over a career on engine failures on the runway and precious little time is spent working real problems that require decision making and CRM skills.

Don't even get me started on this. I have for a long time wondered why on earth airlines do not spend more time on CRM except for the mandatory once a year. Other professions are spending tons on improving their employees "untechnical" skills. Bringing experts in to teach more nuanced topics like man management, time management etc. (or whatever is related to their field) Yet all we get in aviation is the 6 monthly routine and a CRM course. Aviation and the piloting profession is getting left behind. And flying as an occupation is being turned into a glorified blue collar job.

galaxy flyer
10th Feb 2017, 15:53
Jw scud,

Point taken. My airline experience was before the days of LOFT profiles. In USAF, all the profiles were LOFT with the required V1 drills tossed in; i.e. You would abort at V1, taxi back fixed then fly a mission which might conclude with an enroute OEI, approach, miss yo a landing with some associated system problems. In bizav, we just do the checkride evolutions i.e. abort, take-off lose an engine return with hand flown, raw data, ILS and landing. Check box ticked. No LOFT profiles.

Airmann,

Well, there has to be some line in the sand and reducing the screen height allows fairly similar payloads under the two conditions with minimal reduction in safety and perhaps an improvement as JT points out. The USAF, perennially underpowered reduced the screen height, wet or dry, to ZERO. The stop was based on the the nose gear at the last brick or the mains lifting off at the last brick. Interesting on a WAT-limited take-off at Madrid in the summer with 225,000# of munitions on 13.500'

RAT 5
10th Feb 2017, 17:19
The idea of a common screen height seems eminently sensible and difficult to defend the difference. Safety is safety, and if missing the hotel by 15' is OK on a rainy day, why not also on a sunny day? On some runways it is possible to have a higher weight on a wet runway than dry, but some operators don't allow you to do that: you have to check both and take the lower. 35' is safer than 15', perhaps, but on some days 15' is considered not to be. It is or it isn't? Go figure. How to make an easy job difficult. I wonder if any of the muppets at mission control have ever bothered to think about it. Indeed, would they all even understand the question? Sorry; I forgot my anti-cynic pills today.

galaxy flyer
10th Feb 2017, 17:34
It is about probabilities, more take-offs are on dry than wet, so if you more likely to pass that hotel at 15' if we reduced the dry height. Better to have the bigger margin on the more likely scenario.

Max Angle
10th Feb 2017, 17:58
It is about probabilities, more take-offs are on dry than wet,
Which to me seems the problem with the latest (EASA?) missives about using wet performance on a damp runway, we are using wet figures far more often than we used to. In the "old days" wet meant wet, or "would you get a wet arse if you sat down on it?" as an old skipper I flew with used to say. Is the braking action really that much worse on a runway that is slightly discoloured (the definition of damp) than a dry runway that we need to use wet performance?

Airmann
10th Feb 2017, 19:08
The new definition is that its always wet unless its completely dry

Goldenrivett
10th Feb 2017, 19:51
I wonder if any of the muppets at mission control have ever bothered to think about it. Indeed, would they all even understand the question?

If you took off at TOPL weight AND used Maximum thrust AND had an engine failure around your wet V1 AND continued the take off - then your screen height would be 15 feet. How many times a year does that ever happen? (I have hardly ever had to use full thrust ever since leaving the TriStar)

On 99% of occasions we have used FLEX (assumed thrust) when the OAT has been lower than assumed and have therefore had a performance advantage due to the effect of True Airspeed. see: https://flightsafety.org/asw-article/when-less-is-more/"

On the occasions when you use FLEX there will be more clearance than you fear.

RAT 5
10th Feb 2017, 19:58
Is the braking action really that much worse on a runway that is slightly discoloured (the definition of damp) than a dry runway

BA is about stopping; screen height, which is the discussion topic, is about continuing.

Max Angle
10th Feb 2017, 21:43
The reduced runway braking action is the only reason we use wet performance, its all about stopping.

zoigberg
10th Feb 2017, 21:46
Airmann - "The definition is that it's always wet unless it's completely dry"

I would be interested to know where your quote is from. A look at Perf section of EASA 2012 gives ..

EASA CAT.POL.A.105 (d) "For performance purposes, a damp runway, other than a grass runway, may be considered to be dry."

john_tullamarine
11th Feb 2017, 02:29
but I mean I find it strange that they haven't also lowered the dry screen height to 15' and just left it at that.

Unfortunately, I don't have the written rationale behind the wet reduction so I can only surmise. My view is that 35 ft is the standard and, for wet, 15 ft should be viewed as a concession.

The USAF, perennially underpowered reduced the screen height, wet or dry, to ZERO

I wasn't aware of that approach ... making TORR super-serious. Given that the performance numbers aren't precise, although reasonably indicative, what sort of percentage of takeoffs at critical weights throw up some dirt and grass off the runway end ?

if missing the hotel by 15' is OK on a rainy day

.. but keep in mind that we are looking at gross and net flight paths. A serious real in-close obstacle is a worry ... one at 5 miles, not so much ...

On some runways it is possible to have a higher weight on a wet runway than dry, but some operators don't allow you to do that:

..along with the regulator .. refer back to the FAA video link earlier

35' is safer than 15', perhaps,

As GF observed earlier, it's just a number, a line in the sand, albeit based on rational historical data risk assessment. What we have up our sleeves is the gross to net margin .. It really doesn't warrant endeavouring to pull out your tape measure and fret over the last millimetre.

The new definition is that its always wet unless its completely dry

While I have no problem with damp being dry, the end result doesn't appear to have a problem either which way. Have I missed something along the way ?

If you took off at TOPL weight AND ..

We need to keep in mind that the OEI figure is but one of a number of cases considered .. the worst of which defines the TO weight

The reduced runway braking action is the only reason we use wet performance, its all about stopping.

Not quite, if you have standing water, increased rolling and impingement drag (ie go considerations) are very relevant

galaxy flyer
11th Feb 2017, 03:13
JT,

Underpowered is another word for over gross, I guess. We used 2.3% as minimum gradient a significant reduction from the FAR 25 3.0% for quads. Being a quad, AEO take-off run was quite close to OEI BFL, so one saw a lot of runway. The 115% rule provided some protection. That said, climb gradient and obstacle clearance tended to be more restrictive than runway performance, so take-offs didn't usually dust off the overrun.

Airmann
11th Feb 2017, 04:53
Airmann - "The definition is that it's always wet unless it's completely dry"

I would be interested to know where your quote is from. A look at Perf section of EASA 2012 gives ..

EASA CAT.POL.A.105 (d) "For performance purposes, a damp runway, other than a grass runway, may be considered to be dry."

I wasn't trying to be technical. There is a trend in aviation (at least from what I've heard) that a lot of airlines are instructing their crews to use wet even when the runway is damp. I was (trying to) being funny when I said that. I am perfectly aware of what constitutes wet and dry.

Max Angle
11th Feb 2017, 11:24
Not quite, if you have standing water, increased rolling and impingement drag (ie go considerations) are very relevant Agreed but then you are into contaminated performance, as far as I know impingement drag is not considered in wet figures ie. <3mm of water/slush, more than that you have a contaminated runway which is also a 15ft screen of course.

Airmann, in EASA land there is no longer any such thing as a damp runway, there are only three states considered, dry, wet and contaminated. I don't know what has driven this change which came in a few years ago (?) if memory serves but it would be interesting to know. It may be the difficulty in reporting the state and certainly in the UK it has been many years since a runway was reported as damp.

It is now quite common to hear a runway being reported as wet when it is very obviously dry as far as actually stopping on it would be concerned which probably means that wet performance is being used far more than it was before and I am not convinced that safety has been improved.

Edited to add: Having thought about it I guess its obvious that the change has come about to try and reduce runway over runs which is how quite a lot of high speed rejects end up, perhaps there is data to show that these are happening on damp runways and had wet perf. been used it might have been avoided. All good stuff but its a difficult balance, just failing to stop at the end of the paved surface at most airfields is just embarrassing, just failing to clear an obstacle is going to kill you, 15ft is not much of a margin even with the gross/net buffer.

BluSdUp
11th Feb 2017, 12:45
Damp rwy reported all the time in Europe.
With the 737 800 on long trips from some medium rwys, I hate using wet figures when it is realy dry and often grooved.
Why? Because I would like to do a rejected T/ O rather then go airborne with the problem.
So , I end up on the gras doing 4 knot, bu hu.
With the crew we fly with these days, I give it a 50 - 50 chance of survival if we have a engine fail or never mind severe damage and fire.
The notion of, " I would rather go airborne then risk a critical RTO to me is interesting.
Going airborne when rwy limited is no picknic. Rather taxi in for a coffee and quick debrief.

Then again I do live in a simple world.

Goldenrivett
11th Feb 2017, 12:53
which probably means that wet performance is being used far more than it was before and I am not convinced that safety has been improved.
Evergreen International Airlines believes it does.
Aero 11 - Rejected Takeoff Studies (http://www.boeing.com/commercial/aeromagazine/aero_11/takeoff_story.html)

"About 2 percent occur at speeds in excess of 120 kt. The overruns and incidents that occur invariably stem from these high-speed events."

"Approximately 80 percent of the overrun events were potentially avoidable by following appropriate operational practices."

"Using the Boeing data, we initially approached the FAA with a proposal to call a reduced V1 the "decision speed" and treat it as a V1 speed. The flight crew would remove their hands from the thrust levers, and the takeoff would continue. The initial proposed speed was 10 kt less than published V1."

"In late 1992, after we received the Boeing Takeoff Safety Training Aid in draft form, we decided to again seek approval of the "decision speed" concept. This time we chose a speed of 8 kt for a reduction, which added approximately 2 seconds of recognition time. In the worst case the screen height was degraded to approximately 15 to 20 ft."

"We believe that this reduced V1 procedure provides a valuable increase in the safety margin over that provided in the AFM in the event of an RTO."

In order to reduce the chance of an over run from a poorly handled RTO, a reduced V1 (use of wet figures) helps.

BluSdUp
11th Feb 2017, 13:38
Ok , so 50- 50 chance was a bit gloomy. But lets say a fairly large chance of hole in the ground. 5% 10% or 15 % pick Your nr.
And ; no, my present airline is one of the best.
It is an educated guess, considering people do try go-around without power on 777,,,,?
And drill big holes in the Swedish mountains, just because the ADI goes for a loop,,,? Want me to carry on?

There is so much stuff going on these day that we see on OFDM.
You would take the train to Ibiza if you saw it.

With regards to Evergreen Intl.
I that the Yank operation with 5 aircraft according wiky?
Leaders on Flight training and Operations?
We have 2000 departures a day and they have!
Never mind, Flying Club stuff.
Train to Standard, Fly to standard.

zoigberg
11th Feb 2017, 14:28
Airmann - sorry, sense of humour fail from me there!

Speaking for the airline I am currently working for, interestingly I have noticed a few captains opting to look at the wet data when damp. However, in the (4) European airlines I have worked for, I haven't come across pilots being Intstructed to do so.
More noticeable from an anecdotal point of view, is the ATIS still giving wet or damp when the surface is clearly bone dry...... but that's another story

john_tullamarine
11th Feb 2017, 22:22
15ft is not much of a margin even with the gross/net buffer.

Again, gross/net progressively improves whatever you have at runway head as the departure progresses.

So , I end up on the grass doing 4 knot, bu hu.

However, the reality of the dynamics of stopping is that the rate of speed decrease is quite high. The overrun is far more likely to commence at, say, 80-100 kt, than near stopped. On the runway, this speed is washed off quickly without damage (apart from very hot brakes) but, if you hit some rough things at speed, the damage may be significant

galaxy flyer
11th Feb 2017, 22:56
We had duscussion here, eons ago, with Old Smokey et al, (might have involved SSG, JT) on the subject of overruns. First, the risk analyst those classify an overrun as "catastrophic" at, IIRC, something like 70 knots. Second, very few extra knots at the initiation of the high speed reject adds lots of knots at overrun point. Something like 5 knots extra at the first initiation results in 40-60 knots at the runway end.

MarkerInbound
12th Feb 2017, 04:19
FAA AC 25-32

Wet Runway.
A runway is wet when it is neither dry nor contaminated. For purposes of condition reporting and airplane performance, a runway can be considered wet when more than 25 percent of the runway surface area (within the reported length and the width being used) is covered by any visible dampness or water that is 1⁄8 inch (3 mm) or less.
Note: A damp runway that meets this definition is considered wet, regardless of whether or not the surface appears reflective.

Chris Scott
12th Feb 2017, 14:37
I suspect that, in the UK at any rate, this wheeze may have been originated in the mid-1960s by the navigation (performance) department of my own airline, BUA (the one that was later taken over by Caledonian to become British Caledonian).

When you are planing a scheduled operation to Entebbe or Nairobi non-stop from the then 8000-foot runway at Gatwick, using shiny new VC10s, you don't want nearly half the flights to have to tech-stop because the runway is not dry. In the 1970s, our performance instructor (George Seaton) indicated that the airline had come to an arrangement with the ARB (or whomever) that, provided the braking action was good, the screen height could be reduced from 35 ft to 15 ft in the wet-runway case, to accommodate a 10-knot reduction of the V1.

As an aside, George and his colleagues were ahead of the game on take-off performance on contaminated runways. This was arguably a two-edged sword as far as its effect on our jet operations were concerned. I well remember sitting for several hours one wintry day at Manchester on a BAC 1-11 around 1980 trying to explain to my passengers why other airlines' BAC 1-11s were departing to various destinations, whereas we couldn't depart for Gatwick.

RAT 5
12th Feb 2017, 15:19
Chris: drifting I admit, but to follow on your story of the 2 edged sword and differing rules. In early 80's I was in Corfu B732 with a UK airline, departing for EDI from the northly; i.e. towards the mountains. We had RTOW tables from our in house performance guru. No mater how sharp the pencil we could not get the fuel on and had to plan a tech/fuel stop in BHX. Meanwhile, another operator, with the very same ac/ & engines, who was operating charters on behalf of my host airline, loaded up and blasted off for GLA in one foul swoop. How on earth?
I year later I was in a night stop hotel and found a crew from the other airline and asked the question. They did not have RTOW tales and used QRH figures, i.e. balanced field. I asked about the obstacles and mountains. they said this was assessed beforehand and they decided that if anything untoward happened before 1000' they would turn right avoiding the granite visually. Good game.
A few years later I flew for them and they had progressed from B732 - B733. That also was a catalyst for stringent SOP's & RTOW tables. They complained bitterly that 'the fun had gone out of it.' An easy job had become difficult. However, to be fair, they did wise-up and realise that the gung-ho days were over and they needed to join the party.

But this screen height difference I don't get. There needs to be a buffer in missing the terrain, that is a screen height, and a 'net' climb gradient. The net value has an inbuilt buffer in it. It's safe or it isn't. 15' that is. Why, when conditions are better, do you need more margin? If 15' is safe on Monday, why is it not on Tuesday? What does FAA use? I admit I do not know more than I was taught in basic Perf A, and Chris's story might be a key to it. Why then has it not been reviewed and revised? Is the FAA different? If yes, then EU pax on a code share flight with an FAA carrier will be subjected to differing safety standards in their ignorance. That was always the case about FTL's etc before EASA levelled the field. But in global terms is the field level with all the big boys? What rules do the Far East & M.Easy guys use?

BluSdUp
12th Feb 2017, 23:55
John T
If exiting the Rwy end at 80 to 100 kts after an RTO close to V1 is the result, then something must have gone wrong , bigtime.
Would be interesting to see how this crew would have fared if they went airborne??

An RTO shure has to be executed properly , there is a margin but as opposed to many other procedures, not much.
One of the last manual and critical manouvers left in aviation! Have to say I like the Auto brake, try Max every now and then on landing, that gives an idea about the deceleration.

john_tullamarine
13th Feb 2017, 08:59
If exiting the Rwy end at 80 to 100 kts after an RTO close to V1 is the result

I fear you may have missed my point .. perhaps my words were not well chosen ?
The reject is a seriously critical manoeuvre. Of great concern is that, for the stopping bit, the shape of the speed vs time (or distance) curve during the stop is quite steep. Ergo, if there is anything much wrong in the execution of the manoeuvre, and the aircraft ends up a bit further down the runway than intended (for a critical reject limiting runway), it will end up off the runway. Due to the high deceleration at that point, the speed may well be up in the 80-100 kt range with a significant probability of serious hull damage traversing the overrun.

So, yes, something has gone wrong .. a little distance has been squandered. It doesn't take anything bigtime, just squander a little bit of distance ..

For older certification aircraft, particularly, there is scant margin ...

Pugilistic Animus
19th Feb 2017, 02:13
Just remember that the takeoff safety speed must be reached by 35' in both wet and dry condition. As outlined in 14CFR 25

KayPam
28th Feb 2017, 16:30
We had duscussion here, eons ago, with Old Smokey et al, (might have involved SSG, JT) on the subject of overruns. First, the risk analyst those classify an overrun as "catastrophic" at, IIRC, something like 70 knots. Second, very few extra knots at the initiation of the high speed reject adds lots of knots at overrun point. Something like 5 knots extra at the first initiation results in 40-60 knots at the runway end.
I would very much like to see excel calculations supporting this.
Very easy to do, just requires a bit of time.
Double integration with a short Δ, just assume constant realistic values for deceleration and you should have a very clear idea of whether this sentence can be deemed true or false based on simple science.

galaxy flyer
28th Feb 2017, 16:40
I was a economics major, a practical pilot, no engineering or math. Let me know how it works out, please.

KayPam
28th Feb 2017, 17:37
Well I happen to have 5 minutes available
Hypotheses : 4 knots per second during acceleration (or 2m/sē) and +0.4G deceleration
Calculation based on v1 at 120 knots, giving an ASD of X = 1430 m
Then computation for RTO initiated at 125kt : what is speed at length X ?
58kt !!!
Total ASD for rejection at 125kt : 1545m

I am shocked !!

However, 1430m of ASD sounds pretty short, any runway used by commercial aircraft will be longer than that.
Would anyone care to criticize my hypothesis, confirm whether or not the usual ASD (no margins at all) for your A/C will be this short ? (I tried to be representative of a medium jet)

The runway exit speed as a function of rejection speed looks like this :
https://i.gyazo.com/695a1bd56a8a324f85baf5ac6cbf903b.png
There is a beginning to the curve because runway exit speed does not exist if you reject under V1, by definition of V1.
Sharp increase in the beginning, then the steepness of the curve decreases but you're already at high speeds.
There is an end to the curve : the speed you reach at the end of the normal ASD without ever braking.

Next question is how well can an airliner handle a runway exit at higher speeds. Even on grass.

Chris Scott
28th Feb 2017, 20:09
Hi KayPam,

Your sample figures are interesting in supporting galaxy flyer's voice of experience! Actually, however, the acceleration of 4 kt/sec approaching V1 might be a little optimistic at higher weights. Then, of course, the V1 differential we are considering on this thread is typically, IIRC, about 8 - 10 kt IAS, depending on the aircraft type?

galaxy flyer
28th Feb 2017, 23:10
Chris,

Welcome back! It looks like you haven't aged a day since this old thread. I dug it out thus morning over coffee.

http://www.pprune.org/tech-log/327267-would-you-abort-after-v1-11.html

Classic thread

galaxy flyer
28th Feb 2017, 23:19
Kay Pam,

Thanks for your efforts, much obliged. Not to worry about the 1430m runway length, the "test" is a situation where runway available equals accelerate-stop distance of the plane in question. Your numbers would be correct for a mid-weight Global Express business jet, perhaps V1 being a bit high, but in the right area for that 1430m ASDA.

Go to this thread and look at posts 250-254, please

http://www.pprune.org/tech-log/327267-would-you-abort-after-v1-13.html

KayPam
28th Feb 2017, 23:20
Depends how much grass space there is before the first unfrangible object...
If just grass is suggest some pretty reasonable aircraft damage and then most injuries suffered from the actual evacuation but that's just a guess.
Maybe the A/C could get its gear legs stuck in the mud, they could break, leading to serious structural damage or even rupture of the fuselage (which would be catastrophic) ??

Obviously anything built up will be a catastrophy if met at a high speed.

Chris : actually the problem lies in the supplementary distance traveled when you're above v1. Since you're fastest, its when you lose the most runway per unit second (by definition of speed, we call that a Lapalissade in French).
If you were applying full power instead of v1 not during 2 seconds just after v1, but during 2 seconds at 60kt in deceleration, then i'm pretty sure the speed values when exiting the runway would be much more low.
Sure the acceleration would not help neither...

Galaxy flyer : you could abort if you were pretty sure that you can achieve more braking power than the braking power taken into account in the v1 calculation.
I've seen airliners decelerate at +0.8G, that's almost as much as crushing your car brake pedal.
If you had this kind of deceleration (0.8G instead of 0.4) then the few knots would not be a problem.
Obviously what lies beyond the runway should be taken into account as well..

For instance i'm not sure whether full reverse thrust is taken into account ?
I think upslope isn't taken into account as well in the ASD calculation ?
Is it the same for headwind, I think you can only account for 50% of the headwind ?

galaxy flyer
1st Mar 2017, 00:04
Kay Pam

Slope is taken into account. I'm not sure about commercial ops, but USAF we assumed zero wind unless needed then the 50% of the headwind component. In corporate ops (Global Express, pour moi) we never accounted for the wind, but performance was rarely a limiting factor.

Going back I see you used 0.4G deceleration, I think the 0.8G is better, but dependent on the crew fully applying the brakes. The referenced thread spoke of random tests and pilots typically only achieved 75% of max line pressure as the habit patterns of normal landings caused a bit of "laziness" on brake application. One really has to stand on the pedals to achieve full anti-skid braking.

Merci beaucoup

underfire
1st Mar 2017, 07:52
If you are at 15' or even 35' at the end of the runway, you have far bigger issues to be concerned about....

aterpster
1st Mar 2017, 16:38
underfile:

If you are at 15' or even 35' at the end of the runway, you have far bigger issues to be concerned about....
Not if the airplane is using a valid OEI procedure, and all planning and performance requirements were complied with by the operator and the flight crew.

Chris Scott
1st Mar 2017, 17:03
Quote from underfire:
"If you are at 15' or even 35' at the end of the runway, you have far bigger issues to be concerned about...."

You've evidently lived a sheltered life? Try a B707-320B/C at MTOW out of LAX in summer, bound for London. And, by the way, we are talking about the end of the clearway, not the runway...

KayPam
1st Mar 2017, 23:29
What about the 30m margin during the second segment of the go around procedure ?
50m margin leads to being in the third segment !