PDA

View Full Version : Second question Performance V1/Vr Ratio


Engineer
29th Mar 2002, 21:07
In what situation would V1 = Vr and what affect does clearway/stopway have on this relationship

john_tullamarine
30th Mar 2002, 12:47
(a) V1/VR can be made whatever you like within AFM permitted, and specific runway environment, numbers, the higher end limit being V1=VR, ie V1/VR=1.00. . .. .Generally, presuming adequate ASDA, V1 needs to be pushed higher if you . .. .(i) have obstacle problems .. to minimise the TODR . .. .(ii) are going for an overspeed (increased performance) V2 schedule ... to maximise WAT-limited RTOW and/or improve distant obstacle performance .. again by controlling the TODR.. .. .(b) If the intent is to push V1 up, stopway helps a little bit, clearway is not relevant. Clearway becomes of use for runways with reduced ASDA where a reduced V1 can be contained within the ASDA while the clearway extension helps the TODR figures at the lower V1. The TOR restrictions generally limit the utility of clearway.. .. .(c) Sometimes the intention is to go the other way and push the V1 down .. for instance on wet runways or where the overrun is undesirably hazardous ... a matter for company policy and whether the flight standards people carry more weight than the back office bean counters. A problem here is to consider the effect of crosswind on the Vmcg (ie min V1), a matter which the AFM ignores and is not required to be considered (unless the flight standards people prefer to operate conservatively).. .. .Where some pilots get confused is in regard to ill-explained rules of thumb which one sees from time to time for balanced field operations ... generally where some of the related assumptions and restrictions are not well explained. . .. .Most of the time the performance engineer wants to go unbalanced to maximise the scheduled RTOW. The problem which then arises is that pilots often tend to believe the RTOW figures at face value without thinking about THIS particular takeoff and its particular problems on a takeoff by takeoff basis ... a sensible part of the pretakeoff emergency briefing.. .. .There have been numerous discussions on this subject and it would be worth while doing a search and reviewing some of these.. . . . <small>[ 30 March 2002, 07:58: Message edited by: john_tullamarine ]</small>

Engineer
30th Mar 2002, 14:14
In terms of a balance field would the ratio always be 1 if climb limit performance was not limiting?

john_tullamarine
30th Mar 2002, 15:26
Not at all the situation. . .. .The speed ratio for the BFL case will vary from aircraft to aircraft and runway to runway. Depends on factors such at pressure height, OAT, gross weight, wind, runway slope, configuration deviations .. etc etc etc .. most times, the ratio ends up being less than 1.00 for the Types with which I have been involved. . .. .Why are you so concerned about the V1=VR case ? There is no particular significance associated with this case .... except that the ASDR is greater .... and that might well be very significant.

OzExpat
31st Mar 2002, 23:24
I find that V1=VR works nicely for me in the B200. But, then, it don't need a lot of runway, has good off the ground and obstacle clearance performance, without being difficult to control in the event of engine failure or other near-ground emergency. Horses for courses though, just as JT suggests.

john_tullamarine
1st Apr 2002, 02:36
Indeed, Oz .. but, in terms of the question, does that equate to a BFL operation ?

Engineer
1st Apr 2002, 13:27
Correct me if wrong but is the definition of a balance field when the 35ft point =the distance to stop. So if a stopway/clearway involved this ratio is not equal i.e an unbalanced field.

What I am trying to ascertain is in the BF case would V1 = Vr but with the use of clearway/stopway would there then be a ratio involved?

john_tullamarine
2nd Apr 2002, 00:18
Engineer,

Indeed, for a BFL takeoff, TODR=ASDR.

Perhaps you are confusing the (runway) available with the (aeroplane) required distances ?

BFL relates to the aeroplane, not the runway.

A runway may well have TODA=ASDA or, as is more often the case, TODA>ASDA. This has not a great deal to do with a balanced field length takeoff.

However, if the aeroplane AFM calculations, for the conditions pertaining to the takeoff, are scheduled with a selected V1 (or V1/VR - whichever you prefer) such that TODR=ASDR, then the takeoff is said to be scheduled on the basis of a balanced field length. You can, as a result, have a BFL takeoff scheduled on a runway which is several miles long and has a great mismatch between TODA and ASDA... the aeroplane is not particularly interested in all the spare runway which might be available.

If clearway be declared such that TODA>ASDA, then the BFL scheduled takeoff cannot take advantage of the clearway's "extra" distance. To do so would result in TODR>ASDR.

The V1 scheduled for the takeoff is determined from the interaction of a number of competing AFM cases which have to be assessed on a takeoff by takeoff basis. Generally V1 will only be driven to the maximum if you have plenty of runway available to play with. For the BFL case, the numbers will depend on the specific aeroplane and the specific takeoff conditions on the day. I don't think that it is valid to try and generalise... that is to say, for a BFL calculation on several aircraft, the V1/VR takeoff speeds scheduled will, almost certainly, be different.

However, if I have not satisfied the thrust of your question, please do restate it again as you see fit. I am quite sure that many people are not comfortable with what we performance chappies do for a crust (and how we do it) and it is probably worth exploring the subject for the greater benefit.

In the end analysis, though, I wouldn't worry too much about balanced field length calculations. Quite apart from anything else, the great majority of takeoffs are very definitely not scheduled on that basis ... otherwise the accountants would be screaming for the lost kilos.

The only significance of a BFL takeoff is that the AFM usually provides some simplified charts for the BFL case which means that the runway limit can be determined just a tad faster than going through the unbalanced calculations which are more general (and commercially useful).

There is no general advantage in scheduling a BFL takeoff as, on most occasions, the RTOW improves if the takeoff is unbalanced.

Have I helped ? ... or hindered ? .... or have I merely confused myself ?

mutt
2nd Apr 2002, 03:42
Good Morning JT,

Just read your statement... There is no general advantage in scheduling a BFL takeoff as, on most occasions, the RTOW improves if the takeoff is unbalanced.

By scheduling a BFL takeoff you can avail of the speeds provided in the aircrafts FMS, therefore cutting down on crew workload. I would call this a great advantage of BFL takeoffs.

Engineer, the ratio that you are looking for on newer Boeing aircraft is I believe 0.91.

Mutt.

john_tullamarine
2nd Apr 2002, 04:33
Hi Mutt,

A moot point, I guess.

To me, apart from the possibility of a flight standards management consideration at AOC level for standardisation, there is little difference for the pilot in dialling up an FMS page or copying the speed schedule data from an RTOW chart.. and the latter is going to give the higher weight on most occasions.

Then again, you play with newer toys, while I tend to spend much of my time with dinosaurs ....... I am quite happy to use the JB to my advantage .. but I have my ears well stopped to avoid needlessly being seduced on to the rocks by her siren calls ....