Reducing ASDA - effect on operations
Moderator
the performance limit for the clearway is half the takeoff flare distance
Loosely based on this, as I recall, there is an airports standards recommended maximum clearway .. there will be several threads where it's cited. However, unless the takeoff is TORA limited, most times it's not all that relevant.
Loosely based on this, as I recall, there is an airports standards recommended maximum clearway .. there will be several threads where it's cited. However, unless the takeoff is TORA limited, most times it's not all that relevant.
Join Date: Jul 2004
Location: Europe
Posts: 243
Likes: 0
Received 0 Likes
on
0 Posts
JT, TORA limited is a new concept for me (or am I too tired today?). I agree TORA is very limiting. I always like to roll on concrete until lift off 
. Did you mean TODA or am I missing something?
. Did you mean TODA or am I missing something?
Moderator
TORA limited is a new concept for me
.. you and a lot of pilots.
If one lined up a thousand pilots (who hadn't done Perf A or similar training) I venture to suggest nearly all would look blankly at the person who mentioned TORA ...
Quick, heads up brief ..
(a) ASDA tries to set it up so you can stop on the hard stuff if it all turns to custard on the initial part of the journey
(b) TORA tries to get you off the hard stuff BEFORE the end of it - we try not to run on into the mud in the overrun before liftoff (except, occasionally, on 27 at MEL)
(c) TODA tries to get you over the fence (trees, whatever) without hitting it (them)
All of the above can look at the AEO or OEI cases.
My use of "tries" is intentional. There are no guarantees with this stuff in the limiting case .. but the probability of success, as demonstrated by the history, is pretty high.
.. you and a lot of pilots.
If one lined up a thousand pilots (who hadn't done Perf A or similar training) I venture to suggest nearly all would look blankly at the person who mentioned TORA ...
Quick, heads up brief ..
(a) ASDA tries to set it up so you can stop on the hard stuff if it all turns to custard on the initial part of the journey
(b) TORA tries to get you off the hard stuff BEFORE the end of it - we try not to run on into the mud in the overrun before liftoff (except, occasionally, on 27 at MEL)
(c) TODA tries to get you over the fence (trees, whatever) without hitting it (them)
All of the above can look at the AEO or OEI cases.
My use of "tries" is intentional. There are no guarantees with this stuff in the limiting case .. but the probability of success, as demonstrated by the history, is pretty high.
Join Date: Jul 2004
Location: Europe
Posts: 243
Likes: 0
Received 0 Likes
on
0 Posts
Uuh, I think I get it. The way I was seeing was: if you don't make TORA you won't make TODA but thinking twice, that is not necessarily true. I'm one of the few then who's done class A and did not get it on first attempt.
Join Date: Jul 2004
Location: Europe
Posts: 243
Likes: 0
Received 0 Likes
on
0 Posts
I will not delete otr modify the previous post although after some thought I continue to belive that if you don't make TORA you won't make TODA i.e. you are TODA limited.
I'll try to elaborate:
If you don't have enough TODA pavement then you continue rolling on the clearway then rotate and lift off. Even if you manage to make the screen height all your take off flare distance will have been over the clearway which is a no-no (1/2 flare dist limit).
Am I still missing anything?
I'll try to elaborate:
If you don't have enough TODA pavement then you continue rolling on the clearway then rotate and lift off. Even if you manage to make the screen height all your take off flare distance will have been over the clearway which is a no-no (1/2 flare dist limit).
Am I still missing anything?
Moderator
This is a good diversion. You have done Perf A - hence you have a start to a reasonable grasp of the subject - such discussions on PPRuNe are a useful vehicle to knock off the rough edges.
For the majority of the younger pilot set, I suspect that performance doesn't go much past learning how to look up the RTOW tables on the flightdeck whilst on the ramp. Certainly, if that not be the case, then a lot of folk were asleep during their performance lectures. For the airline with which I started out, the Ops Engineers did all these lectures and the folk got quite a bit of over and above information .. albeit that many never quite got their heads around the nitty gritty.
If you don't have enough TODA pavement ....
TODA generally comprises the whole runway PLUS the clearway. Clearway usually is not pavement and, most definitely, is NOT for mixing with heavy aircraft bogies. It is not a case of having enough TODA - rather, you limit the TOW such that the TODR ≤ TODA. If you don't do this you are outside the AFM rule book requirements.
then you continue rolling on the clearway
I really hope that folk don't want to do this - against the rules and pretty foolhardy .. you MUST plan the numbers on the basis of being airborne some distance PRIOR to the end of the declared TORA.
then rotate and lift off
liftoff MUST occur not further along than that point in the declared TORA which will constrain the first half of the airborne distance to screen to be located over the TORA ie you MUST always be airborne BEFORE the end of TORA - the runway sealed bit.
all your take off flare distance will have been over the clearway which is a no-no (1/2 flare dist limit).
No - you are missing the important distinction - only a portion of the flare will be located over clearway and then only if you are running with an unbalanced takeoff schedule - TORA doesn't extend into the clearway bit. For a balanced field approach to the problem, clearway is irrelevant and doesn't come into the discussion at all.
However, please do continue the discussion - the aim is to air these sorts of things especially for the benefit of the newchums who often don't get any of this stuff in their training.
For the majority of the younger pilot set, I suspect that performance doesn't go much past learning how to look up the RTOW tables on the flightdeck whilst on the ramp. Certainly, if that not be the case, then a lot of folk were asleep during their performance lectures. For the airline with which I started out, the Ops Engineers did all these lectures and the folk got quite a bit of over and above information .. albeit that many never quite got their heads around the nitty gritty.
If you don't have enough TODA pavement ....
TODA generally comprises the whole runway PLUS the clearway. Clearway usually is not pavement and, most definitely, is NOT for mixing with heavy aircraft bogies. It is not a case of having enough TODA - rather, you limit the TOW such that the TODR ≤ TODA. If you don't do this you are outside the AFM rule book requirements.
then you continue rolling on the clearway
I really hope that folk don't want to do this - against the rules and pretty foolhardy .. you MUST plan the numbers on the basis of being airborne some distance PRIOR to the end of the declared TORA.
then rotate and lift off
liftoff MUST occur not further along than that point in the declared TORA which will constrain the first half of the airborne distance to screen to be located over the TORA ie you MUST always be airborne BEFORE the end of TORA - the runway sealed bit.
all your take off flare distance will have been over the clearway which is a no-no (1/2 flare dist limit).
No - you are missing the important distinction - only a portion of the flare will be located over clearway and then only if you are running with an unbalanced takeoff schedule - TORA doesn't extend into the clearway bit. For a balanced field approach to the problem, clearway is irrelevant and doesn't come into the discussion at all.
However, please do continue the discussion - the aim is to air these sorts of things especially for the benefit of the newchums who often don't get any of this stuff in their training.
Join Date: Jul 2004
Location: Europe
Posts: 243
Likes: 0
Received 0 Likes
on
0 Posts
I did perf A during my frozen ATPL days and I would have to remove a thick layer of dust before daring to touch those files. That may explain it. After that though, I have digested a few CDs and PPTs.
Sorry, temporary disconnection between fingers and mind. I meant "If you don't have enough TORA pavement ...."
I see I was not successful in conveying -possibly due to the initial mispelling- that the rolling on the clearway part was purely a theoretical exercise.
I'll try that again. My understanding is that TORA limited means you are unable to lift off before the end of the pavement (TORA). What I reckon is before you are TORA limited you are already TODA limited.
Example: TORA=TORR+1m => TODA limited (You are TODA limited before being TORA limited).
Although I could be wrong I am far from seeing it.
If you don't have enough TODA pavement ....
I see I was not successful in conveying -possibly due to the initial mispelling- that the rolling on the clearway part was purely a theoretical exercise.
I'll try that again. My understanding is that TORA limited means you are unable to lift off before the end of the pavement (TORA). What I reckon is before you are TORA limited you are already TODA limited.
Example: TORA=TORR+1m => TODA limited (You are TODA limited before being TORA limited).
Although I could be wrong I am far from seeing it.
Moderator
Peace be upon you, my son .. the aim is to clear the cobwebs for the benefit of whomever might chose to follow the thread ... absolutely essential that we all accept that none of us knows more than a few of the answers ... and can learn more, day by day.
TORA limited means you are unable to lift off before the end of the pavement (TORA).
Not quite ... you have to be off the pavement AND halfway to screen height by end of the TORA. If that is the limiting T/O calculation then, indeed, you are TOR-limited. Note that, if you needed to go further than the TORA then you are OVER your TOR limiting weight and in trouble. Unfactored TODR is the distance at which you get to screen height.
What I reckon is before you are TORA limited you are already TODA limited.
Not at all the case - Type/runway dependent. Not a generalisation which can be made
Example: TORA=TORR+1m => TODA limited (You are TODA limited before being TORA limited).
Why should that follow ? You can easily be TOR limited and not TOD limited for a given runway ? Similarly, you may be under the TOR limiting weight, as well as under the TOD limiting weight and routinely be ASD limited, for instance ... or limited by some other consideration.
Although one gets a bit of a feel for a given Type after a while, philosophically, ALL the calcs are run and the lowest weight comes up trumps for RTOW.
Absolutely no reason why any case should, necessarily, have the inside running. Having said that, for a given Type, one will see a skewing of the results such that the limitation may well be ABC more often than not .. but the next Type may be quite different in its results.
This is one of the problems which confuses the student greatly when many training organisations attempt to condense the subject matter down into rules of thumb ... which often need a bunch of caveats added for the discussion to make sense.
TORA limited means you are unable to lift off before the end of the pavement (TORA).
Not quite ... you have to be off the pavement AND halfway to screen height by end of the TORA. If that is the limiting T/O calculation then, indeed, you are TOR-limited. Note that, if you needed to go further than the TORA then you are OVER your TOR limiting weight and in trouble. Unfactored TODR is the distance at which you get to screen height.
What I reckon is before you are TORA limited you are already TODA limited.
Not at all the case - Type/runway dependent. Not a generalisation which can be made
Example: TORA=TORR+1m => TODA limited (You are TODA limited before being TORA limited).
Why should that follow ? You can easily be TOR limited and not TOD limited for a given runway ? Similarly, you may be under the TOR limiting weight, as well as under the TOD limiting weight and routinely be ASD limited, for instance ... or limited by some other consideration.
Although one gets a bit of a feel for a given Type after a while, philosophically, ALL the calcs are run and the lowest weight comes up trumps for RTOW.
Absolutely no reason why any case should, necessarily, have the inside running. Having said that, for a given Type, one will see a skewing of the results such that the limitation may well be ABC more often than not .. but the next Type may be quite different in its results.
This is one of the problems which confuses the student greatly when many training organisations attempt to condense the subject matter down into rules of thumb ... which often need a bunch of caveats added for the discussion to make sense.
Join Date: Jul 2004
Location: Europe
Posts: 243
Likes: 0
Received 0 Likes
on
0 Posts
After dusting off some material I found out what you already pointed out which is that 1/2 the air distance belongs to the TOR.
In this case it my example was inapropriate. I have always assumed that the field limited weight was the smaller of the TODA limited and the ASDA limited weight.
At least I have an excuse to go through all that stuff once more:
Love and peace.
PS: I wonder what DOB I put on my profile which has induced you to think I could be your son. I'll check that too.
In this case it my example was inapropriate. I have always assumed that the field limited weight was the smaller of the TODA limited and the ASDA limited weight.
At least I have an excuse to go through all that stuff once more:
Love and peace.
PS: I wonder what DOB I put on my profile which has induced you to think I could be your son. I'll check that too.
Moderator
I have always assumed that the field limited weight was the smaller of the TODA limited and the ASDA limited weight.
almost .. the minimum of TOR, TOD and ASD limited weights. One of the reasons that this is not understood, generally, is that the majority of OEMs combine some of the AFM data and often TOR is not shown explicitly. Good British machines, on the other hand, tend to let it all hang out in all its glory ...
I'll check that too.
I was tempted to come back with a cheeky response but that probably would lower the tone of things .... best regards to you, good sir, and best of good fortune with your career.
almost .. the minimum of TOR, TOD and ASD limited weights. One of the reasons that this is not understood, generally, is that the majority of OEMs combine some of the AFM data and often TOR is not shown explicitly. Good British machines, on the other hand, tend to let it all hang out in all its glory ...
I'll check that too.
I was tempted to come back with a cheeky response but that probably would lower the tone of things .... best regards to you, good sir, and best of good fortune with your career.
Guest
Posts: n/a
Just had a look at Google Earth for WAW. Has anyone else noted the humonguos displaced threshold on the recip RWY 29?
Suggests to me that there's an obstacle forcing the TOCS back up RWY 11.
Can see a railway line crossing and a built up area beyong which appears t include hi-rise housing.
Sir George Cayley
Suggests to me that there's an obstacle forcing the TOCS back up RWY 11.
Can see a railway line crossing and a built up area beyong which appears t include hi-rise housing.
Sir George Cayley
Prof. Airport Engineer
Join Date: Oct 2000
Location: Australia (mostly)
Posts: 726
Likes: 0
Received 0 Likes
on
0 Posts
Greetings Sir George. You've stimulated me to fire up Google Earth and take a look at this runway. I haven't checked the obstacles as you have, but I noticed something else.
I surmise that the full length of WAW runway 11/29 was used in the old days; and I measured this as 2800m (which is a nice round credible number). I also observe the concrete pavement slabs over 200m at each end of the full runway, and this is the normal position that an airport engineer would place them covering the start of the takeoff roll (since concrete is better than asphalt in supporting slow moving or stopped aircraft. It costs more than asphalt, so after 200m, the concrete is replaced by asphalt. That's why so many aprons are concrete while their runways are asphalt). I also observe the taxiway layout is such that there is a taxiway at each end of the full runway length. Could this be a cavalier [sic] military design without regard to civilian niceties?
So why did they shorten it now, and publish the odd ASDA and TORA/TODA figures?
Looking at the 'stopway' at the 29 end of 11/29, and then using the magic of Google Earth to tilt the picture [shift-down arrow keystroke] until it is almost flat, the very undulating nature of the terrain becomes clear. And the original designers have clearly done nothing by way of earthworks to provide a level runway. They simply laid it on the ground and it goes up and down as the ground goes up and down. Just east of the main (new/proper) runway end line and the big piano keys, the runway dips sharply. This dip is just beyond the declared TORA/TODA, and I am sure this bit of runway doesn’t meet ICAO requirements for runway or strip gradients (especially runway end gradients). Someone has picked the problem up on audit, and made the airport adjust their declared distances.
My guess is that the runway AND strip have then been arbitrarily terminated at the new runway end at 2300m (because of the gradient issues beyond that). There is still plenty of good runway and plenty of clear runway strip to the east BUT it doesn’t meet the ICAO rules except if used as a stopway. Thus the TORA = TODA = 2300m. And ASDA = 2597m.
The 2597m ASDA stops when the runway changes to concrete. It complies with ICAO because there is good runway and cleared strip out to that distance (just a bit steep gradient wise). I suspect that beyond it, the concrete is in really poor condition and not suitable for aircraft. I see a couple of concrete slabs have been replaced at WAW, and I have experience elsewhere of old Soviet military airfields and their teeth-rattling concrete slabs. . . .
My guess is that these odd declared distances are a clever bit of juggling by an astute engineer to squeeze the most out of a difficult situation.
Cheers,
Overrun
I surmise that the full length of WAW runway 11/29 was used in the old days; and I measured this as 2800m (which is a nice round credible number). I also observe the concrete pavement slabs over 200m at each end of the full runway, and this is the normal position that an airport engineer would place them covering the start of the takeoff roll (since concrete is better than asphalt in supporting slow moving or stopped aircraft. It costs more than asphalt, so after 200m, the concrete is replaced by asphalt. That's why so many aprons are concrete while their runways are asphalt). I also observe the taxiway layout is such that there is a taxiway at each end of the full runway length. Could this be a cavalier [sic] military design without regard to civilian niceties?
So why did they shorten it now, and publish the odd ASDA and TORA/TODA figures?
Looking at the 'stopway' at the 29 end of 11/29, and then using the magic of Google Earth to tilt the picture [shift-down arrow keystroke] until it is almost flat, the very undulating nature of the terrain becomes clear. And the original designers have clearly done nothing by way of earthworks to provide a level runway. They simply laid it on the ground and it goes up and down as the ground goes up and down. Just east of the main (new/proper) runway end line and the big piano keys, the runway dips sharply. This dip is just beyond the declared TORA/TODA, and I am sure this bit of runway doesn’t meet ICAO requirements for runway or strip gradients (especially runway end gradients). Someone has picked the problem up on audit, and made the airport adjust their declared distances.
My guess is that the runway AND strip have then been arbitrarily terminated at the new runway end at 2300m (because of the gradient issues beyond that). There is still plenty of good runway and plenty of clear runway strip to the east BUT it doesn’t meet the ICAO rules except if used as a stopway. Thus the TORA = TODA = 2300m. And ASDA = 2597m.
The 2597m ASDA stops when the runway changes to concrete. It complies with ICAO because there is good runway and cleared strip out to that distance (just a bit steep gradient wise). I suspect that beyond it, the concrete is in really poor condition and not suitable for aircraft. I see a couple of concrete slabs have been replaced at WAW, and I have experience elsewhere of old Soviet military airfields and their teeth-rattling concrete slabs. . . .
My guess is that these odd declared distances are a clever bit of juggling by an astute engineer to squeeze the most out of a difficult situation.
Cheers,
Overrun
Thread Starter
Join Date: Jul 2007
Location: Gdansk
Posts: 38
Likes: 0
Received 0 Likes
on
0 Posts
Hi Overrun,
I am not sure about the gradients being non-compliant, since departures regularly use the full lenght distance (2800m) when departing on 29. However, I am sure the fact that the 29 threshold is at the bottom of the slope worsens the obstacle situation for approaches on 29 and departures on 11, and I believe this is the main reason for the threshold displacement for approaches on 29 and why on 11 the TORA and TODA are limited to 2300m (clearly obstacles not such a big issue for ASDA
)
Also I believe the ASDA for 11 is less than 2800 because the remaining distance is a RESA (can't get the RESA further out east because of the mound which acts as blast protection).
I am not sure about the gradients being non-compliant, since departures regularly use the full lenght distance (2800m) when departing on 29. However, I am sure the fact that the 29 threshold is at the bottom of the slope worsens the obstacle situation for approaches on 29 and departures on 11, and I believe this is the main reason for the threshold displacement for approaches on 29 and why on 11 the TORA and TODA are limited to 2300m (clearly obstacles not such a big issue for ASDA
) Also I believe the ASDA for 11 is less than 2800 because the remaining distance is a RESA (can't get the RESA further out east because of the mound which acts as blast protection).
A very interesting thread. I read it as an inexperienced 738 driver who passed (and enjoyed) Performance during the ATPL theory exams, but always wondered how it relates to the real world.
OverRun's explanation of WAW certainly makes good sense. Thanks for that.
OverRun's explanation of WAW certainly makes good sense. Thanks for that.
Join Date: Sep 1999
Location: ME
Posts: 5,505
Likes: 0
Received 0 Likes
on
0 Posts
RWY 11 TORA 2300 ELEV 110M SLOPE -0.33
RWY 29 TORA 2800 ELEV 103M SLOPE +0.27
This is taken from a database which uses the difference in runway end elevation divided by length to get the slope. As its not one of our regular airports, no one has actually walked the runway. Maybe this airport proves that the concept of just using runway ends isn't always the correct approach.
Mutt
RWY 29 TORA 2800 ELEV 103M SLOPE +0.27
This is taken from a database which uses the difference in runway end elevation divided by length to get the slope. As its not one of our regular airports, no one has actually walked the runway. Maybe this airport proves that the concept of just using runway ends isn't always the correct approach.
Mutt
Moderator
Maybe this airport proves that the concept of just using runway ends isn't always the correct approach.
Which is why some up us never use this other than for runways with reasonably even slope ... just a matter of checking out the centreline longitudinal section survey data .. usually available.
The need is to overlay the actual aircraft performance on the section survey so that one doesn't, eg, run the ASDA with a totally inappropriate slope ...
Which is why some up us never use this other than for runways with reasonably even slope ... just a matter of checking out the centreline longitudinal section survey data .. usually available.
The need is to overlay the actual aircraft performance on the section survey so that one doesn't, eg, run the ASDA with a totally inappropriate slope ...
Join Date: Jun 2004
Location: Australia
Posts: 1,843
Likes: 0
Received 0 Likes
on
0 Posts
Mutt's comment - "Maybe this airport proves that the concept of just using runway ends isn't always the correct approach" is very valid, and J_T (as usual) beat me to the response.
In my performance work, I take a conservative approach to what I term "Composite Slope" runways. I stand by for incoming klak for being too conservative, but this is the technique that I employ in generating RTOWs -
For each runway, I run 3 analyses for each Takeoff consideration, i.e,
(1) The RTOW for the Mean Slope,
(2) The RTOW for the greatest UP slope, and
(3) The RTOW for the greatest DOWN slope.
The applicable limit (Accelerate-Go, Accelerate-Stop, MBE, etc.) is then the LEAST of the limits found. Many pilots don't notice, but one management pilot asked my why, for the same runway, the Slope printed on the RTOW was different for Full Length Vs Intersection departure - The answer; composite Runway Slope. Now throw in a few other factors such as CWY and SWY slope differing from the "Main" Runway slope (The EPWA case), and the runway becomes interesting.
My "mind set" began when once holding at the takeoff point waiting for an aircraft (B747) taking off in the opposite direction (Actually Mutt, it was one of yours, definately no criticism of you). It was (and is), a significantly humped runway, both ends the same elevation, thus Zero mean slope, but significant UP slope to the centre, followed by significant DOWN slope to the end. The said aircraft faced a degraded Takeoff performance, due to the actual Takeoff being UP slope instead of Zero, and a significant detriment to Accelerate-Stop due to DOWN slope if such a thing had happened (it didn't). The main gear cleared the runway end by just a few feet.
Yes Mutt, I do agree with you, the concept of just using runway ends isn't always the correct approach, there is an alternative, however conservative it may be.
Regards,
Old Smokey
In my performance work, I take a conservative approach to what I term "Composite Slope" runways. I stand by for incoming klak for being too conservative, but this is the technique that I employ in generating RTOWs -
For each runway, I run 3 analyses for each Takeoff consideration, i.e,
(1) The RTOW for the Mean Slope,
(2) The RTOW for the greatest UP slope, and
(3) The RTOW for the greatest DOWN slope.
The applicable limit (Accelerate-Go, Accelerate-Stop, MBE, etc.) is then the LEAST of the limits found. Many pilots don't notice, but one management pilot asked my why, for the same runway, the Slope printed on the RTOW was different for Full Length Vs Intersection departure - The answer; composite Runway Slope. Now throw in a few other factors such as CWY and SWY slope differing from the "Main" Runway slope (The EPWA case), and the runway becomes interesting.
My "mind set" began when once holding at the takeoff point waiting for an aircraft (B747) taking off in the opposite direction (Actually Mutt, it was one of yours, definately no criticism of you). It was (and is), a significantly humped runway, both ends the same elevation, thus Zero mean slope, but significant UP slope to the centre, followed by significant DOWN slope to the end. The said aircraft faced a degraded Takeoff performance, due to the actual Takeoff being UP slope instead of Zero, and a significant detriment to Accelerate-Stop due to DOWN slope if such a thing had happened (it didn't). The main gear cleared the runway end by just a few feet.
Yes Mutt, I do agree with you, the concept of just using runway ends isn't always the correct approach, there is an alternative, however conservative it may be.
Regards,
Old Smokey
Join Date: Sep 1999
Location: ME
Posts: 5,505
Likes: 0
Received 0 Likes
on
0 Posts
O_S for regular airports we actually follow your method. For example in Manila the final portion of he runway slopes up more than average, so we use this value and accept the resulting 3000 kgs payload drop.
The older 747 is generally limited by all engine go, hence this is one of the reasons that you see them almost scraping the lights.
Mutt
The older 747 is generally limited by all engine go, hence this is one of the reasons that you see them almost scraping the lights.
Mutt