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oceancrosser
16th Mar 2015, 13:15
I have come across Airbus field performance data, that details performance for a dry runway and a "wet (with DRY check)". Can anyone shed any light on what Airbus means with that?

vilas
16th Mar 2015, 13:27
On wet and contaminated runways the screen height is reduced to 15Feet from 35Feet for dry RW. That can give higher TOW in wet conditions instead of restricting it. So regulations do not permit TOW to increase beyond dry TO weight. This is known as dry check.

High_Expect
16th Mar 2015, 13:30
Always wondered why it's acceptable to erode a set safety margin just because it is wet. Surely the 35ft screen height is that for a reason?

Australopithecus
16th Mar 2015, 13:40
It seems that it is based purely statistics and the balance of risks. But that of course disregards climates which are equally likely to be wet as dry.

A 15' screen height is ludicrous, in my opinion. At my base there is a sea container showroom across the road from the clearway, with containers stacked up to 79' . How would a 15' screen height work there I wonder?

oceancrosser
16th Mar 2015, 14:25
Thanks vilas, so another case of juggling numbers in a "legal" way.

FlyingStone
16th Mar 2015, 14:52
A 15' screen height is ludicrous, in my opinion. At my base there is a sea container showroom across the road from the clearway, with containers stacked up to 79' . How would a 15' screen height work there I wonder?

Screen height only covers the clearway, so you are at least somewhat airborne at the end of clearway. What you are saying is you have an obstacle in the initial takeoff flight path, so treat it accordingly...

FE Hoppy
16th Mar 2015, 15:27
close in obst with wet perf can be interesting as the Net take off flight path starts at 35' but the aircraft may only be at 15'.

Citation2
16th Mar 2015, 19:30
During certification , use of reverse is mandatory on a wet runway and not a dry one. This might give a higher MTOW on a wet , due to actual lower accelerate stop distance.

However Accelerate stop distance on a wet runway is the higher of : ASD dry , ASDwet , ASD wet(N-1)

Although Actual accelerate stop distance on a wet would be lower than a dry due to the use of reversers ( lower distance means higher MTOW), DRY accelerate stop distance would be retained if higher .

Therefore all speed calculations , optimum and balanced V1 would be calculated to comply with the dry if more limiting.

john_tullamarine
16th Mar 2015, 22:28
with containers stacked up to 79'

As observed earlier, treated either as a first or second segment obstacle and would be quite limiting I suggest.

Net take off flight path starts at 35' but the aircraft may only be at 15'.

One should read the NFP as starting at the end of the TOD. The head height and NFP obstacle clearance height is then built into the analysis. Agree that it does read strangely, though.

JammedStab
17th Mar 2015, 03:33
Same for the ATR where numbered codes were given for what was actually causing the takeoff limitation.

"Limitation Codes

0 Dry Check: The wet/contaminated performance may be better than the
dry performance due to the definition of takeoff distance for
wet/contaminated runways having a screen height of 15 ft instead of 35 ft. In this case, dry performance must be used (regulation) and are therefore
automatically displayed on the wet/contaminated chart with the limitation
code 0"

Speedwinner
17th Mar 2015, 06:51
Let me get this straight: the 35ft and 15ft are based on a really tight takeoff , zero stop margin and full thrust and no more space right? In all other cases we will cross the obstacle much higher right?

FE Hoppy
17th Mar 2015, 10:10
Our Canadian cousins produced a nice document about this topic.
https://www.tc.gc.ca/media/documents/ca-opssvs/700-016.pdf

Page 34 for wet.

FlyingStone
17th Mar 2015, 10:15
Let me get this straight: the 35ft and 15ft are based on a really tight takeoff , zero stop margin and full thrust and no more space right? In all other cases we will cross the obstacle much higher right?

And engine failure at Vef of course...

john_tullamarine
17th Mar 2015, 10:48
Let me get this straight: the 35ft and 15ft are based on a really tight takeoff , zero stop margin and full thrust and no more space right? In all other cases we will cross the obstacle much higher right?

The situation is not quite that pessimistic ...

If there is no engine failure and the aircraft is operated sensibly, heights will exceed the OEI analysis by a significant margin .. ie the normal situation.

For the OEI case, the takeoff analysis looks at a bunch of cases which are calculated ... one of these will be critical and determine the limiting weight .. noting that the AEO case with its factor can be the limiting case.

If OEI TODR case is limiting then the aircraft flight path may approximate the 35'/15' screen height at the end of the TODR (which will be less than or equal to the TODA) .. otherwise, the aircraft can be expected to be somewhat higher.

From this point, the net flight path as calculated has to clear the actual obstacle profile by 35'. The OEI flight path, actually achieved, will exceed the net profile by (at least) the delta between net and gross.

As the takeoff progresses, this delta provides an increasing pad for actual obstacle clearance.

JammedStab
17th Mar 2015, 22:49
The situation is not quite that pessimistic ...

If there is no engine failure and the aircraft is operated sensibly, heights will exceed the OEI analysis by a significant margin .. ie the normal situation.



One does wonder how much clearance there would really be in a worst legal case scenario when heavy takeoffs in some aircraft a few years back seemed quite low at runway end with all systems operating.

john_tullamarine
18th Mar 2015, 00:37
One does wonder how much clearance

The typical sort of heavy departure with a significant V1/Vr split using maximum clearway can raise the eyebrows for onlookers ...

One can't see the runway here http://www.strangemilitary.com/images/content/167533.JPG but, presuming it to be in frame, you get the idea ...

All fine and according to Hoyle. Providing that the conditions reasonably replicate the certification testing the takeoff is sensibly repeatable. From this point, the obstacle clearance gets progressively better away from the runway due to the better gross compared to net gradients. Indeed, fleet data will provide a small additional benefit.

The need is for the ops engineers to know their business, do it accurately and well .. and for the crew to fly iaw the book.

For very close in obstacles (first segment or early second segment) the clearance can be minimal and approach 35ft ... from there on it gets better.

No Fly Zone
23rd Mar 2015, 03:37
Disclaimer: to be clear, I do NOT fly jets in the Airbus/Being class. That said, a lot of this discussion appears to emphasize all engines operating and one engine out (or lost at a critical moment) that pushes performance to the absolute limits of 'legal.' I also appreciate that many, if not most performance parameters include a significant fudge factor.

All considered, virtually all of my training repeatedly emphasized the concept that "Legal is not necessarily smart, safe flying." Again folks, I do not fly these birds and or in their usually tight flight schedules. In my environment, the pilots make the final call. I've never pushed those numbers to the absolute limits and to my knowledge, nor have my colleagues.

Perhaps the original inquiry was only an academic inquiry, yet some of the responses suggest that AB and B pilots routinely push these numbers to the hilt. And perhaps I've missed something. In any case, I won't fly that close to the edge of the performance envelope. If I've missed something, please remind me. Flame if you must and PM is also welcome.

john_tullamarine
23rd Mar 2015, 04:32
Ah .. the distinction between commercial and private operations .. in the latter one can impose whatever level of conservatism might be seen as desirable .. in the former, the aim is to make a buck and the pilot needs to have a sensible reason for imposing "extra" levels of conservatism.

Realistically, although the "low over the end of the runway" scenario looks impressive, the exercise is quite repeatable and not necessarily high risk.

galaxy flyer
23rd Mar 2015, 14:16
NFZ,

I'd guess you don't fly in and out of Aspen, then. Or, any field length limiting runways like TEB. or any number of short, country airports. Truth is, if I'm guessing right, loads of business jet class planes are operated like the EVA Air departure, they just don't realize what OEI ops look like. And quad take-offs always look more "spectacular" because they are quads and the engine loss is only 25% of total thrust.

I just did recurrent and a WAT-limited take-off from KTEB can be eye opening.