Difference in Wet and Dry Screen Height
 

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

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!

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?

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.

Some really good opinions from Old Smokey, Oldebloke, John T. etc. circa 2006. dry/ wet screen height [Archive] - PPRuNe Forums

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.

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..

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.

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.

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.

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.

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.

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 may provide some useful thoughts for the discussion

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.

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.

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'

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.

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.

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?

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

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.