smartt1982
Topic Author
Posts: 226
Joined: Sat Nov 24, 2007 11:17 pm

Unbalanced Field Length, Any Difference?

Sat Jun 09, 2012 3:25 pm

With being presented an unbalanced field length, do we actually get any extra benefit if we have more of one thing than the other e.g. clearway/stop way.

Do the figures (V1) look any different if we are still constricted to keep it balanced i.e. the distance for the stop case still needs to be the same as the distance for the go case?

Does this impede/restrict aircraft performance because you cant take advantage of the extra bit of stopway/clearway if you only have one of them, as in you could take that extra weight if you for example defiantly knew you wouldn’t have to stop on the runway but not in the other case?
 
tdscanuck
Posts: 8572
Joined: Wed Jan 11, 2006 7:25 am

RE: Unbalanced Field Length, Any Difference?

Sat Jun 09, 2012 7:33 pm

Quoting smartt1982 (Thread starter):

With being presented an unbalanced field length, do we actually get any extra benefit if we have more of one thing than the other e.g. clearway/stop way.

Yes. Stopway counts as clearyway (since a stopway is obviously clear) but not the other way...so having more stopway gives you the benefit of the stopway (more stopping distance available) *and* the benefit of the equivalent clearway (since you can fly over it). Having more clearway only gives you an advantage on obstacle clearance but not on stopping distance.

Quoting smartt1982 (Thread starter):
Do the figures (V1) look any different if we are still constricted to keep it balanced i.e. the distance for the stop case still needs to be the same as the distance for the go case?

If you're constrained to stay balanced you'll be forced to the more restrictive of the two. It should be a lower V1 for extra clearway than for extra stopway.

Quoting smartt1982 (Thread starter):
Does this impede/restrict aircraft performance because you cant take advantage of the extra bit of stopway/clearway if you only have one of them, as in you could take that extra weight if you for example defiantly knew you wouldn’t have to stop on the runway but not in the other case?

Yes. To take full advantage of a clearway you need to do an unbalanced field calculation (if your limit is obstacle clearance or stopping distance). If your limit is Vmcg, tire speed, brake energy, etc. then it may not make any difference.

Tom.
 
smartt1982
Topic Author
Posts: 226
Joined: Sat Nov 24, 2007 11:17 pm

RE: Unbalanced Field Length, Any Difference?

Sat Jun 09, 2012 8:45 pm

Again, thanks Tom,

Quoting tdscanuck (Reply 1):
If you're constrained to stay balanced you'll be forced to the more restrictive of the two. It should be a lower V1 for extra clearway than for extra stopway.

Is there anyway to know if the calculations are based on a balanced calculation or not? All I know is that we go into a regulated take off weight chart/table for the runway we are using
 
tdscanuck
Posts: 8572
Joined: Wed Jan 11, 2006 7:25 am

RE: Unbalanced Field Length, Any Difference?

Sat Jun 09, 2012 9:40 pm

Quoting smartt1982 (Reply 2):
Is there anyway to know if the calculations are based on a balanced calculation or not?

Not from just looking at the speeds. If you have access to the all-engine go/one-engine stop/one-engine go distances you can tell (for example, the Boeing Onboard Performance Tool or Boeing Laptop Tool provide this data). Or you can ask the person doing the performance calculations. But I don't know of a way, a priori, to know just by looking at the speeds.

Quoting smartt1982 (Reply 2):
All I know is that we go into a regulated take off weight chart/table for the runway we are using

Best bet is to talk to the people who did the chart. If it's a Boeing stock chart, I believe they default to balanced field and I believe Airbus defaults to unbalanced field, but I'm not positive about that.

Tom.
 
mandala499
Posts: 6458
Joined: Wed Aug 29, 2001 8:47 pm

RE: Unbalanced Field Length, Any Difference?

Sat Jun 09, 2012 10:13 pm

Quoting tdscanuck (Reply 3):
Best bet is to talk to the people who did the chart. If it's a Boeing stock chart, I believe they default to balanced field and I believe Airbus defaults to unbalanced field, but I'm not positive about that.

If "Improved Climb Take Off" is unbalanced, then Airbus' is unbalanced, as their default take off tables already include improved climb. The Airbus tables is "maximum load possible for a given runway length", either TOD limited, or ASD limited, or climb gradient limited, and there's a limit code in the table depicting which limitation it's at for whichever scenario. But the TOD limit or ASD limit is based on TORA, not TODA or ASDA.
Can't be bothered to look at them at the moment as bed is waiting for me, so, don't blame me if the above are way off!

Mandala499
When losing situational awareness, pray Cumulus Granitus isn't nearby !
 
tdscanuck
Posts: 8572
Joined: Wed Jan 11, 2006 7:25 am

RE: Unbalanced Field Length, Any Difference?

Sun Jun 10, 2012 12:04 am

Quoting mandala499 (Reply 4):
If "Improved Climb Take Off" is unbalanced, then Airbus' is unbalanced, as their default take off tables already include improved climb.

Yes, an improved climb takeoff is unbalanced.

Tom.
 
smartt1982
Topic Author
Posts: 226
Joined: Sat Nov 24, 2007 11:17 pm

RE: Unbalanced Field Length, Any Difference?

Sun Jun 10, 2012 10:27 pm

Quoting tdscanuck (Reply 5):
Yes, an improved climb takeoff is unbalanced

Is that the only/majority of the time were there is an unbalance?
 
tdscanuck
Posts: 8572
Joined: Wed Jan 11, 2006 7:25 am

RE: Unbalanced Field Length, Any Difference?

Sun Jun 10, 2012 10:58 pm

Quoting smartt1982 (Reply 6):

Quoting tdscanuck (Reply 5):
Yes, an improved climb takeoff is unbalanced

Is that the only/majority of the time were there is an unbalance?

That would depend, a lot, on individual airline procedure. From a performance standpoint that's probably the most common reason to do an unbalanced takeoff but I don't know what actually drives it most in practice.

Tom.

Who is online

Users browsing this forum: Bing [Bot], strfyr51 and 26 guests