Cap56, I'm not too sure why you feel this is such an emotional issue.

As far as I can tell, re-reading the thread, no-one has claimed that the "V1 such that ASDA=TODA=BFL" definition is "wrong". It's simply not "unique".

It's hardly the only TLA in aviation that has multiple meanings.

oh, and just out of curiousity, what's a "B-level"? is that something a "Brit" would know about??

Good folk ..

These sorts of threads are wonderful to see as they encourage people to be forthright in challenging each other's not always absolutely correct understanding of this and that. At the end of the day, it is our hope that people challenge their own ideas and, occasionally, change them. It is a truism that, if you ask a question of 20 flying folk ... often you will get 30 slightly (and sometimes wildly) varying answers.

A couple of points ..

(a) several of the posters in this thread are known to be very experienced, practising performance and/or flight test engineers. Mutt, for example, looks after the numbers for a LOT of big aeroplanes flown by a well known airline ...

(b) BFL calculations, generally, do NOT give maximum RTOW (unless we are talking about aircraft whose AFM provides ONLY BFL data - some do, most don't). Depends on the runway and the aircraft. However, what BFL calculations DO do is to give the easiest and quickest calculation.

BFL calculations generally give a good weight quickly.

... and, as has been observed earlier .. there are no sheep stations riding on these sorts of questions .. so there ought not to be any need for rising temperatures ?

Methinks that the paranoia and emotion arising here is not so much as it is a performance issue, but a VERY commonly asked interview question.

If there is any residual curiosity out there from a purely performance perspective, refer to Mutt's last post.....I concur.

If it is the interview thing as O_S suggests, then it's a shame that interviewers would apparently rather have a stock, learned-by-rote, answer than a response indicating that the interviewee may actually have done some thinking for themselves and, in fact, be better informed.

There is no single right answer in engineering, and even when the rules appear to mandate one it is a foolhardy man who closes his eyes to the real world behind the regs. I, for one, am very wary of people who know the regs, procedures, etc., by heart, but appear to have no understanding of the topic itself.

(Not intended at anyone in this thread, before someone takes offence; I have seen other cases, though, where the regs are quoted blindly out of context)

For the Airbus view of this subject, with diagrams; see the excellent document
Getting to grips with aircraft performance

MFS' comments on interviewing unfortunately are right on the mark. Many interviewers know not much more than the interviewee and, on occasion, far less... which makes things hard for both parties... particularly as we get older and suffer fools less politely than we did as youngsters ....

On the one hand, an imperative (often seen with pilot interview processes) is to weed out those who ought not to have got to the interview stage (except for the misplaced value of their innovative CVs)... hence the use of canned question and answer ...

On the other hand I recall an interview for an engineering job many years ago where the principal interviewer (who became my next boss and from whom I learnt a lot ...) asked not one straight forward question ... all the questions were thought provoking.

As he confided some months later over a coffee .. he hadn't been terribly interested in the specific answers so much as the thought processes which the to and fro discussion on each question revealed.

Mind you, I did find one question .. along the lines of "tell me a bit about supercharged gas turbine engines" and the subsequent discussion on engine design and operating envelopes interesting ...

Mutt,

You consistantly make inordinately good sense, but I'm a little puzzled by the rationale of your company policy spelled out in your recent posting -

Is it true that you don't take any credit for Clearway / Stopway ? Perhaps your Company's operations are on good long runways and CWY/SWY credit is not needed. Where possible we do not use them if unwarranted, but will immediately do so on shorter runways where an operational advantage exists. (This requires a policy statement to pilots re the use of FMC speeds. Certainly, we adjust FMC speeds for MEL / Contaminated runways). I'm interested to know how you can justify this penalty (although the good long runway explanation might be all that's needed).

When you state that you don't account for optimized Vspeeds, are you referring to their 'fine tuning' due to CWY/SWY, or to Vspeeds optimized for V2 overspeed / Improved Climb which is not considered by the FMC ? In the case of the former, OK, I'm with you, you don't use CWY/SWY, but if it is your policy to not use the latter, I cannot see the justification. On a recent departure from JED at 43°, we squeezed on another 2600 Kg of payload in using 'Improved Climb' (I hate that Boeingism). These higher speeds most certainly require over-writing of the FMC balanced field speeds.

Genuinely interested to see your reply, we're always looking at Company policy improvements for maximising commercial advantage, or similarly towards operational simplification if there is no commercial penalty involved.

Old_smokey,

Will have to answer this at a later date, the Emerald Isle and a keg of Guinness beckons!

Or better yet, if you are back here in October, we can discuss it over a beer or two.

Cheers.

Mutt. http://www.stopstart.fsnet.co.uk/smilie/guin.gif

Mutt,

Thanks. I look forward to both, the answer and the beer. The October part is very much in the hands of the forces of darkness that control my roster.........Will try to twist their tails a bit.

Regards,

Smokey

Can I resurrect this thread?

Can we have your reply Mutt to Old Smokey's question?

Cheers.

:ok:

Yikes, thanks for dragging this up SR71, I had forgotten about it!

The easiest way is to deal with each item individually, I will add that we operate an extremely diverse fleet so my answers will refer to our aircraft in general and not particularly every aircraft type.

Stopway Not accounted for. Goes back to B707 days so I don’t know the rational behind it.

Clearway (a) Generally speaking, a large percentage of our operations use a generic takeoff chart for all runways in a particular airport, takeoff weight given for 4 temperatures based on the worst runway at that airport. Using this, there is no point in using clearways. (b) Some takeoff chart programs don’t allow clearway’s. (c) Airports in home country are long.

Increased V-speeds. Big NO NO, crews have no desire to use higher than required V-speeds. Initial aircraft/home base takeoffs were field length limited, therefore improved climb wouldn’t work!

The goal is to keep takeoff calculations as simple as possible, basic takeoff charts are used with the QRH/FMC V-speeds with minimum corrections. The desire to achieve this is usually greater than the desire to get the highest amount of “thrust reduction”, or additional payload!

Mutt.

Cheers Mutt,

One thing (amongst others!) bothers me though with analyses that don't exploit the maximum available distances for "go-ing" or "stopping".

As Old Smokey says, there will be a performance implication, but there will also be a reject implication.

In any situation where a spread of V1 speeds is applicable, if I am working with normal speeds as opposed to improved speeds, by forcing me to go flying for a Vef > V1 in this particular case, rather than reject, I am being denied the opportunity to take advantage of the fact that ASDA(V1_normal) < ASDA(V1_improved) whereupon there may be ample distance available to reject even after V1?

The inherent conservatism (i.e. the ability to stop within the remaining distance available) in ASDA that exists in the case of a rejection at normal V1 speeds exists up to V1_max doesn't it?

Isn't it the case that when departing from a runway using normal V-speeds but that affords you the option of increased V-speeds, technically it is the increased V-speed V1 that is the latest safe point for a reject or am I missing something?

FWIW, a quick peek at RW 24 @ PRG in our FCOM shows a ~30kt spread of V1 speeds for normal versus increased V-speeds T/O. At an average of 150kts, thats probably worth a good 500m...

:ok:

I've always believed that the reason we use Balanced Field Length figures is that it simplifies calculation (as mentioned earler by A.Whittington) and also presentation.

In an airline with multiple destinations to airports with multiple runways - doing a detailed runway analysis for each and every combination of runways/temps/pressures etc would obviously be impractical.

Using a Balanced Field Length takes a 'Standard Runway' template and moves it from runway to runway where possible, thus avoiding multiple individual analyses.

This standard template is one which gives MAX performance (Max RTOW) from the aircraft at ISA conditions, and allows for corrections where necessary (temp/amb press/wind etc).

In the event a runway cannot give max RTOW using BFL then a specific analysis would have to be done.

As Mutt says, the catch (IMHO) arises when you put the BFL template for a small jet onto a 14,000' RWY. You may in fact then only be 'lookig at' the first few thousand foot of the runway (the BFL template) and you are potentially ignoring thousands of feet which are available beyond that.

This means that you could be led to believe you must make the V1 decision far earlier than actually possible in reality.
In current methods of presentation you often don't know if you are using BFL or not. Perhaps it would be a good idea if all Perf Charts had this annotated on the page?

By the way...all of the above could be utter bullcrap. I am simply stating my own (self deduced) understanding of the situation. Please correct me if I'm wrong.

Idunno

A very clear explanation of the practical (as opposed to exam) use of BFL, with one small inaccuracy - the BFL figures in the manual do not give the MAX performance, just a simplified one that is fairly close to max. If you apply your template to a particular runway and it is not long enough, you may be able to "make it fit" by doing a full performance analysis taking into account stopway, clearway etc. Obviously if the runway is a Balanced Field (exam definition), then there will be no benefit.

The penalty for this simplification is exactly as has been described - you ignore all the runway beyond the "template" which could be used for stopping after an engine failure or accelerating to an increased V2 for obstacle clearance. But if your operation is one where you are rarely "performancearily challenged";) , then this is a penalty that many companies are happy to take in the pursuit of simplicity = less chance of silly cock-ups!

Happy Flying

BJJ

Not trying to be provocative or anything, but to me, if we consider a normal TO with a V1 of 135kts and we have a catastrophic failure/fire at 105kts ( << V1) but went flying anyway and subsequently wind up in the bushes, thats a silly cock-up isn't it?

The situation with PRG above is the same isn't it? We have our catastrophic failure/fire at V1 = 135kts but could reject anywhere up to 165kts.

In this case, if we wind up in the bushes, we say the guys did the right thing?

Incongruous.

My last performance course was woefully inadequate.

IMHO, we live in a day and age where we shouldn't have to rely on a BFL analysis template for the various fields we operate to. The number-crunching ability of your average desktop is phenomenal. The only reason more don't complain about the implications of the simplifications is probably to do with the fact that not many of us (myself included) fully appreciate how our PERF manuals are compiled.

:uhoh:

SR71

In your example, if there is enough runway top stop from 165 instead of 135, then there is also plenty of margin to continue from 135 as well. The type of "silly cock-up" I was referring to was someone trying to do the full calculations from the graphs in the AFM and getting it wrong. A friend of mine had a very interesting departure in a 707 after getting Fahrenheit and Celcius mixed up!!

However, you are absolutely right about modern PC's. One of the companies I fly for regularly have just purchased a system that has a worldwide runway and obstacle database and all the performance for all the different aircraft they operate, which is being loaded onto each aircraft's laptop! This is definitely the way forward.

"Aim for the stars and you may just miss the trees"

SR71,

There are many different ways to accomplish a safe takeoff calculation; your goal seems to be to use the Maximum V1 so that you only take to the air at the last possible moment. This is a perfectly valid reason but it doesn’t appeal to everyone.

Recently observed on a B777 flight, mid weight where the choice of Flaps 05 or 15 were available, Captain chose Flaps 15, when asked why; he replied that he was concerned with brake fires with higher V-speeds. He therefore wanted to abort from the lowest possible V-speed; his preferred choice would be Minimum V1.

We have operated the A300 for about 20 years with optimized V-speeds, crews hated the idea that for a 1000 increase in takeoff weight, there could be a 30 knot increase in V-speeds, they campaigned for years to have the speed rational changed to Balanced.

So basically there isn’t one easy answer to satisfy everyone. Some airlines do actually give crews min/bal/max V-speeds and allow them to make their own decisions, however around here the predominant mindset that was developed for the Classic B747’s and carried over to all fleets is for lower speeds to aid the accelerate stop.

Mutt.

Mutt,

I suppose I haven't explained myself that clearly.

Your aforementioned 777 skipper...I concur. I'd always seek to use the TO configuration that kept the V-speeds to a minimum depending on the TOW for exactly the reason he suggests.

However, whatever type of TO I am accomplishing, I'd like to know the max V1 at which an abort is possible bearing in mind the actual ASDA.

Presumably your skipper above isn't aware of whether or not it is the AGD or ASD that is limiting the BFL to a particular value, whereupon once V1 arrives, his decision is determined for him.

But isn't forcing me to go flying at a V1 determined from a balanced field analysis where it isn't the ASDA that is limiting imposing an artificial constraint on me unnecessarily? And with potentially the worst of consequences...

Sure, we have loads of margin in the case of an abort, but we've forced the decision to commit to getting airborne before we needed to.

Returning to my scenario above concerning PRG where we are considering a normal TO with V1=135kts and where an increased V2 speed TO for the same TOW gives V1=165kts, as BizJetJock says, if I have a catastrophic failure at 135kts, from a performance point of view, theoretically getting airborne doesn't present me with a problem, but why take the risk in this scenario?

In this instance, I know that I could quite safely abort from anything up to 165kts.

I think this sensible. I would still always use the lowest speeds which allowed me to get airborne for the TOW on the day, but am I missing something when I suggest that if a particular TO is possible at a particular TOW via a range of V-speeds, technically the option to abort always exists up to the max V1 possible?

:ugh:

Surely all the airline interviewer wanted to hear was something along the lines of:

"An airport has a runway at the end of which is a clearway and a/c must reach their screen height at the end of the clearway. Some of the clearway includes a stopway, which is purely there to support an a/c after a rejected take-off.

The maximum distance from the far end of the runway to the end of the clearway is never to be more than 150% of the actual runway length; however, if only the stopway is considered rather than all the clearway, the distance available to meet screen height is equal to the distance available from starting the take-off to stopping if the take-off has to be rejected. Such a case, where ASDA=TODA, is referred to as a 'balanced field'."

KISS!

balanced field calculations do not take into account obstacles so how do you account for required clearance when using bf tables?

Correct FE Hoppy, BFL does not consider obstacles. Operator's policy and techniques will vary, but for the Companies / Aircraft types that I'm responsible for as Performance Engineer, there are 2 approaches.

Primarily, I only use BFL for the production of 'General Takeoff Charts and Tables', this takes care of the 'Field Limits', i.e. Accelerate-Stop or Accelerate-Go to the screen height (nominally 35 feet). Now, from the end of the Runway, obstacles for the 1st and 2nd segment climbs are evaluated, and the 2 associated 1st / 2nd segment climb limits found. Now we have 3 limits, namely (1) The Field Limit, (2) the 1st segment limit, and (3) the 2nd segment limit. The MBRW is then the lesser of the 3 limits found. This may lead to some pretty horrendous limits, particularly when the 1st segment obstacles are 'close in'.

Alternatively, a technique is described in the Operations Manual/s for when the 1st/2nd climb limits are too restrictive. Steadily reduce the Field length in increments, thus making the distance to the obstacle greater, and the required gradient less. Thus, whilst the Field limit is reducing, the obstacle limit is increasing. Keep on reducing the Field length until the Field Limit and the Obstacle limit are as close as possible to each other, and Voila!, you've done a reasonable job of optimising the takeoff weight. The big 'sufferer' in all of this of course is the accelerate-stop limit (because you've been reducing the Field length), which need not have been so as the Accelerate-Stop is unaffected by obstacles.

The techniques described here are crude, but effective in achieving reasonable takeoff weights whenever the RTOW is invalidated. It's a brief and incomplete description, but it will keep you safe.

Take Care,

Old Smokey

My first question is "do all the crews realise exactly what you're doing".

I'd be concerned that someone who *thinks* they know (but doesn't) will note you've used a shorter BFL than the available runway, and think "aha, I can takeoff from the intersection, rather than taxiing all the way to the end of the runway...."

Mad (Flt) Scientist,

To answer your second question first, the Operations Manuals that I've produced and alluded to PROHIBIT the use of intersection departures when General Charts / Tables are used.

To respond to your first question, "do all the crews realise exactly what you're doing", probably not. Whilst well written Operations Manuals are produced, and recurrent performance refresher courses etc. are carried out, there's still a significant number of pilots 'out there' who, having passed Performance 'A', promptly put it into the 'tick in the box' category and hope that they never need to use it. Methinks that the number of very basic performance related queries / responses on Pprune's pages are indicitave of this.

The best that we can do is produce the best possible performance material, training syllabi, and guidance to trainees, and hope that their professionalism is such that they would studiously apply it.

Em, why was the question asked in the first place? Flying Clog claims to be an airline captain - is this subject not comprehensively covered as part of the ATPL study?

FJJP.

To answer your question which I quote -
I respond with my own previous quote -
Old Smokey

Old_smokey,

What type of aircraft are you talking about?

Mutt.

Old_smokey,

If you were just talking about the basic BFL charts in an operations manual I would agree with you that they do not account for obstacles, as they are not airport/runway specific.

However the Boeing software methodology reduces the takeoff weight for the obstacle and then provides a balanced field solution. This data is then presented as a limiting takeoff weight and the speeds can be obtained directly from the FMS.

All companies don’t use this method; I have another electronic AFM that even though the input selection is "balanced", obstacles will “unbalance” the output.

It is therefore imperititive that crews KNOW exactly what their ops engineers are trying to do, this can only be achieved through adequate training and company procedures.

Mutt.

SR71,

Still interested in knowing how you actually brief the takeoff procedure. Using the lower V1 of 135kts, with the knowledge that you can safely abort at 165 kts. Do you brief your PNF that your maximum abort speed is 135 or 165?

Mutt.

The most limiting sea level runway that I have personally operated from is FUK.
Takeoffs to the south (runway 16) are obsticle limited, whereas takeoffs to the north (runway 34) were field length limited...with TriStar -500 equipment.
Our routes were such that field length limited takeoffs were always performed on runway 34 (wind permiting of course) and therein was a problem.
The takeoff data was for a balanced field, yet I had no doubt whatsoever that trying to stop from anywhere near V1 would positively result in an overrun, something I personally was not about to tolerate.
So, I always had a short discussion with the other flight crew, and suggested that a reduction of V1 was appropriate, thereby avoiding an overrun (if required to stop) yet allowing the continued takeoff, with a lower screen height achieved, should an engine fail at the most unfortunate time.
Wet runway takeoff data (reduction of V1) was used, directly from the takeoff data supplied, and was derived from Air Canada manuals.
Seemed reasonable to me...and still does.

IMO, whatever it takes to keep you safe, and avoid that certainly unpleasant incident/accident.

...but of course using the wet V1 left you about 10kt closer to a Vmcg that had been calculated without crosswind and might even have put you under your 'real' Vmcg on the day.

Which would have caused the utterance of a singularly appropriate expletive considering the ICAO code of the aerodrome in question.....

In HMFC's VC10s, we used either aerodrome-specific regulated take-off graphs, or 'general' balanced field graphs if RTOGs weren't available. For normal conditions a mid-range V1 was used; this varied for contaminated ops or for wet RWs.

Reduced thrust ('factored power') take-offs were OK using most general balanced field graphs; however, on some RWs it was necessary to use max chat - and to treat the locals to the 'silent and serene' sound of the mighty Vickers Whisperjet!

Mutt,

Agreed, on the one hand I was referring to basic Operations Manual BFL Charts where obstacles have to be considered separately.

On the other hand I was referring to those occasions when a published RTOW is invalidated (Usually by the appearance of a new obstacle), and in the interim period (often only a matter of hours) before the computer programme can generate temporary RTOWs, crews are required to use the "General Performance" data from the Operations manuals. This procedure is well spelled out to our crews, as I alluded to in a response to Mad (Flt) Scientist with the words "Whilst well written Operations Manuals are produced, and recurrent performance refresher courses etc. are carried out, there's still a significant number of pilots 'out there' who, having passed Performance 'A', promptly put it into the 'tick in the box' category and hope that they never need to use it. Every effort is made to convey to the pilot group the reasoning required to effect this, and it is gratifying that there is a growing awareness of performance appreciation and appropriate application in my company (This did not exist before, and I take some delight in having turned the tide).

The computer programmes that I've developed (from Manufacturer's AFM and Performance Manual data of course) rarely use the Balanced Field principal, as we regularly use Clearway and Stopway in MOST normal operations which, except in a few cases, are rarely equal. The major de-equalising (is that a word?) factor of course, is the presence of obstacles where the Optimised Continued Takeoff/Obstacle limit does not conflict with the separately computed Accelerate-Stop limit, the lesser of the 2 being the limit. This cannot be the case when forced to use "General Charts" when Balanced Field data is used in the instance of the RTOW being invalidated.

I mentioned elsewhere that I was primarily referring to the B777, but these same general principals, with their own idiosynchrasies, apply to the other aircraft that I do performance work for.

Sometimes I think we say the same things, but use different words.

Regards,

Old Smokey

Thread appears to get bogged down from time to time but is, nonetheless, a very important message to get across to the newchums periodically ..

Consider -

(a) the main aim is to come up with numbers to provide the operating crew with data unlikely to cause embarrassment - the bean counters may take a contrary view, of course

(b) the next aim is to be able to defend the numbers and processes in court

(c) the operating crew, however, must know enough about the basis of the numbers to operate in a manner compatible with the analysis.

(d) sometimes this results in pilots with not much more than a very elementary "do it this way" knowledge right through to folk who are competent to do the whole thing .. end user performance work is hardly rocket science .. one just needs to be methodical, disciplined, have good obstacle data and housekeeping.

(e) whether the analysis is balanced or unbalanced really is unimportant, provided that the numbers don't put the aircraft (on paper) off the side/end of the runway, into the hill, etc.

(f) balanced calcs are great for general charts to be used in association with climb gradient data by the pilot when the RTOW tables are invalid. Quick and dirty answer to the immediate problem without taking forever to do the sums ..

(g) except for folk who are used to using the AFM, it generally is not feasible for the line pilot to pull out the book on the ramp and do the full analysis. When one considers that the main problem is defining the actual obstacles, it would be pointless for the pilot to attempt an optimised calculation anyway.

(h) flight standards considerations are extremely important. While the bean counters have their part to play, often the difference between a balanced and unbalanced calculation is not significant so there may be the opportunity to use the simpler data to make it a bit easier on crew workload, etc. A similar argument can be made for facilitating the use of computer based cockpit assistance .. FMCs and the like.

(i) generally, unbalancing will provide extra payload so it is a good thing if the alternative is to leave a bunch of people behind at the terminal

(j) unbalancing to optimise the numbers may result in speed schedules which are not comfortable for the pilot .. considerations of high speed aborts become very relevant from the viewpoints of standardised practices and corporate risk control

.. and the list can be extended to further detail without too much effort ....

Well....... its a neat summary but its an engineer's approach. The list is too long, you got down to (j) before you got bored, the pilots lost you at (c).

The problem isn't pilots who don't know what they're doing, they're likely to follow SOPs. The problem is not pilots who really know what they're doing, they will consider all the variables. The real problem, as always, is pilots who think they know what they are doing and don't, always the majority.

The answer can only be,

1. Operators, prepare the SOPs with the advice of specialists. Ask for what you want, the specialist will give it to you.
2. Pilots, always follow SOPs, do not deviate. You ain't paid to think.

Steps back and awaits incoming.

Mutt

I've never questioned the standard SOP...until now!

Alex,

Hey, who are you calling a chimp?

;)

Not you, unless you have a sense of humour. Have you noticed how little free thought pilots actually have?

Alex,

.. actually, I'm an engineer who was perverted by being a pilot .. and a pilot who, likewise, suffered at the hands of boring engineers ... come to think of it, at times I'm not quite sure what I am. Mind you, many others have no hesitation in suggesting various answers to that question .. but that's entirely another matter ..

In fact, the poor guy who drew the short straw and got me to train as an intake airline FO didn't know what had hit him .. fortunately he had a good sense of humour and perservered so here I am still ...

But I concur with your thoughts ... 100 percent.

Well, maybe 99.9 percent.

A lot of sim training work suggests to me that the "legend in his own mind" is a menace and that the better breed of pilot is one who

(a) follows SOPs unless there is a very pressing and sensible reason to do otherwise

(b) doesn't do non-standard things without a good deal of thought and consideration, being well aware that a lot of armchair philosophy went into developing most SOPs.

(c) always considers the need to be able to explain his/her action(s) at a later time

No offence meant chaps! I was being slightly 'tongue in cheek' but there was a serious point behind it.

... but I'm improving .. this time I ran out of puff at (c) ...