Monitoring takeoff acceleration

[mutt]

Taken from an excellent post by arcniz over in the R&N forum.

THIS - real-time acceleration analysis during each and every departure roll - is the 21'st century way to solve the problem, even if weighbridges and other passive controls are added as a further means to assure load limits.

The informal discussion here has clearly pointed out that quite a number of factors can influence the ability of the bird to depart her runway in the time and space available. Weight, elevation, temperature, humidity, thrust, wheel friction, winds, turbulence, runway surface contamination, etcetera. A large number of variable but potentially decisive factors affect the launch outcome. Most are hard to know precisely and uncontrollable once the roll begins. The choices mid-roll devolve to power, airfoils, and reject/braking.

Departure performance is pre-calculated with an expected weight, thrust, slope, winds, temps, etc. The calculation, as done manually, creates a few numbers such as MTOW and V-speeds. But the same information and the same calculation can just as easily create a complete moment-by moment profile curve, expressed as speed vs distance or acceleration vs time or roll distance vs time from throttle up.

Having this computed curve of expected performance at hand, it is the most natural thing in the world for a little bit of electronics to compute a moment-by moment 'actual performance' during the roll from airspeed, surface radar, gps, wheel spin, acceleration, or all of the above. By continuously comparing the projected performance with the observed values, the resulting real-time all-encompassing magical performance number would provide a very true comparison of expected versus actual progress in safely unsticking from mother earth. And the information - at least for gross deviations in performance - would be available and useful early in the roll, making an informed take-off abort possible before the uncertainty of stopping makes scrubbing the takeoff a terribly hard decision.

This evolving launch data could be plotted in detail on the FE console, if present, but a few lights would probably serve the purpose up front. Green, amber, red and brown might be appropriate.

What this bit of kit would do is add method and precision to just what a pilot does now: monitor the progress of the takeoff run and determine whether it "feels" right or not. The difference is that it would add a degree of accountable precision to a seat-of-the-pants process and work just as well in cold dark places, in rain and snow and other cases where crew sensory perception is constrained.

I dont want to distract from the actual crash discussion, so posted it here.....

Any comments?

Mutt.

[The African Dude]

Great stuff - after all, didn't this kind of creative thinking produce the aeroplane to start with?

:cool:

[Mad (Flt) Scientist]

It sounds like there is an unreasonable expectation of precision in the proposal; there's significant scatter in all test data for takeoffs, and more so operationally I suspect. I'm not sure what it's going to tell the crew usefully that they don't already know, without being set to a threshold that generates false warnings routinely.

[JABBARA]

The difficulty is, from brake release point or time to V1, the acceleration is not linear. This is because drag and thrust changes with speed. So acceleration at each point of ground run will be varying. If it were linear, it would be very simple and helpful to furnish the flight deck with an instrument comparing the actual accelaration with a preset expected or calculated acceleration value. If the actual acceleration is not in the slot, rejecting the take off well before V1 would be much safer.

In the air force, we used to check the distance markers on the sides of runway and compare it with current IAS to ensure the airplane is in the slot of acceptable minimum acceleration.

[Flight Safety]

I thought I would re-post these 3 links discussing some development work that's been done on TOPMS (Take Off Performance Monitoring System), and the issues raised by the work that's been done so far.

NASA TOPMS research (http://oea.larc.nasa.gov/trailblazer/SP-4216/chapter6/ch6-5.html)

University of Saskatchewan thesis on TOPMS system (http://library.usask.ca/theses/available/etd-04242003-171306/)

NASA TOPMS cockpit display development (http://nix.larc.nasa.gov/info?id=EL-1996-00060&orgid=1)

Fundamental to making this system work, is that it MUST be able to accurately predict (within an acceptable margin of error) if the aircraft will reach the calculated V1 speed at the calculated distance down the runway, at the present thrust settings. As a secondary function, the system should be able to predict if the aircraft can reach the calculated Vr speed before the end of the runway, at the present thrust settings.

It seems to me that there are three primary areas of technical development that must be done to make this system work.

1. Aircraft position on the runway must be known by the system to a fairly high degree of accuracy, and the runway parameters must be known by the system.

2. Current speed and acceleration must be known, and predictable changes in acceleration must be known with speed increase, thrust change, current aircraft configuration, etc. This concept of dynamic prediction is easy to grasp, but the reality of actually making it work is quite complex.

3. An acceptable display and pilot information and warning system must be developed.

The problem with the method used now, is that once calculated V1, Vr and thrust settings have been obtained, the pilot accelerates down the runway watching a buildup to these speeds, without any direct measurements of how much runway has been used while reaching those speeds.

(Epiphany moment) :D

In thinking this through, I recall in the old Jimmy Stuart movie about Charles Linberg's famous crossing of the Atlantic, that they put a painted white stick in the ground next to the runway at the point where the Spirit was supposed to reach Vr. I wonder if the NASA approach for a TOPMS system is too complex in trying to dynamically predict the down runway speed and distance of the aircraft during a takeoff roll, based on present up runway performance.

Maybe it would just be simpler for a TOPMS system to create computer generated "sticks" along the runway at set distances in a display, with expected speeds at each distance "milestone", after the takeoff parameters have been entered. Current speed data is easy to obtain, and current distance data down the runway should be easy to obtain. Then all the system has to do is compare and display expected speeds with actual speeds, as each of these runway "milestones" is reached. You would quickly know whether you were accelerating along the expected performance curve or not, and the reason for a slower than expected takeoff could be determined after the abort. This system would be MUCH simpler to design, compared to a dynamic performance predicting system.

[Loose rivets]

An interesting one for me. Thirty years or so ago I returned from the US with an agreement with the inventor of a device that I saw immediately could be turned into an ‘acceleration achieved’ display—operating in real time.

The old Colonel in Texas had spent most of his life lecturing on blind flying…in the later years these lectures became highly regarded reminiscences at Randolph AFB etc., of attempts to make people believe that instruments were necessary at all. He was also the holder of several aviation patents.

I quickly drew up plans and proposals for the appropriate government offices, but was disappointed when they advised me that they were already financing ( 50% ) research on such a system. A BAC 1-11 was routinely charging up and down some runway using Doppler—the beam being squirted onto the concrete—and, I think, a dual needle on the ASI.

It all came to nowt, which was prbably a good thing for my finances, but a very interesint time. The old gent had done many things, including throwing hooded pigins out of aircraft (they paraglided) and using a stethescope to hear pings from an echo bounced off the fog covered surface...high tech days.

[bullshot]

We already have all the gear on board for an effective takeoff acceleration monitor and alert system...IRS for accel achieved + Runway data in the FMC database (to calculate accel required)
All it requires is some clever team to write up the program. Speed tapes have a trend vector - if required acceleration to 80kts is not achieved this vector arrow could start flashing red and a timely stop could be made. It would be inhibited above 80kts. Get on to it Guys!
There have been many T/O performance accidents that have not involved engine failure - the DC-8 60 series (was it ONA or Capitol?) comes to mind that started T/O with the parking brake set comes to mind, and what about the recent 747 at YHZ? - although I don't know the details of that one.
Some time ago there was anecdotal evidence of an interesting T/O by a major 747 carrier at a far-eastern airport. Thrust was set and the a/c trundled off - albeit slowly, down the strip. Things looked pretty tight but it eventually staggered into the air and dragged its @rse all the way to flap retract, when climb thrust was selected and - the a/c came rapidly to life and accelerated away nicely! For some reason the set thrust had been nowhere near t/o thrust and it had gone unnoticed.
We need a simple T/O acceleration monitoring and alert system

[DOVES]

Once upon a time there was a parameter to be checked during the take off run.
It was by first seventies, when I was flying LA CARAVELLE;
The "Time to 100 Knots".
Perhaps it's time to resume it?:ok:

[Jagbag]

In Jaguars on hot and high days we had a speed check at every marker (300m) and at the middle marker if we didnt cross above a certain speed, it was mandatory to reject. we had a couple of airfields at very high altitudes where take offs were really hair raising.

The similar for landing at the middle marker we had to be less than 100 Kts or we would deploy the tail chute.

There are accelerometers available linked to the INS which could be checked by the PNF about 2-3 times and if the acceleration drops below a certain value than one could be warned. In addition this would help during deceleration on landing.

[Flight Safety]

The reason I think a "speed at distance" system would be easier than a "dynamic performance predicting" system, is the basic equation a=f/m.

In this equation, acceleration is changed by either an unexpectedly low thrust level (including unexpected drag) or an unexpected weight. Depending on the level of sophistication desired in a dynamic predicting system that might cue pilots for actions during takeoff (such as increasing thrust, etc), the system has to accurately figure out whether there is a mass problem, or a thrust problem. This would be complex and the system could never be wrong for safety's sake. Then the system would have to dynamically change and display its prediction based on performance changes made by the pilot.

The "speed at distance" system would be much simpler to design, certify and implement, as there would be no prediction (after the initial speed at distance plots were generated), and no action cues would be generated by the system for the pilot (thus no possibility of false warnings or false action cues). The pilot (PNF)would simply read the displayed numbers and use his judgement instead (and follow SOPs etc).

A HUD display generated by this system would be possible as well. :O

[RatherBeFlying]

I like this because you can simply add the calculated/measured/interpolated time to V1 column to the takeoff charts.

Start the stop watch at throttles up and abort if V1 is not achieved by that time plus a tolerance.

If acceleration is insufficient when the time is up, then you do have more runway ahead at the expected time and have more distance to stop and less speed to get rid of;)

This is a better deal than being 30 kt. short of V1 while rapidly consuming remaining runway and beginning to wonder what's going on:uhoh:

In a glass cockpit a/c, the time remaining could be represented as diminishing line hanging below V1 that would flash appropriately as the decision point is approached and sometimes exceeded.

An alternative representation could be based on the familiar remaining circular seconds symbol we see on movies that would sit in the middle of the attitude display -- we're still on the ground and won't be flying until past Vr.

One could also choose a speed such as V1-10 so that possibly distracting displays would be gone when it's time to fly the thing.

Start the clock ticking automatically when the throttles go past what's normally used to begin taxi.

[White Knight]

Time to 100 knots on the 'bus works very well!!

The 332 - set 1.03 EPR, start the clock and voila - 30 secs to 100 kts.
The 343 - set 50% N1, start the clock and voila - 30 secs to 100 knts.
The 345 - set 1.05 EPR, start the clock, and also voila - 30 secs to 100 kts.

This works for every take-off that I've tried it on:ok:

PS - RR Trents on the 332!

[6100]

I don't think there is much rocket science in this. It is simply time vs airspeed. You have all the tools you need in the cockpit. An ASI and a stopwatch. All that is missing is the performance data from the manufacturer.

When I was flying C130's in the military we used this system to great effect when doing max effort takeoffs from limiting strips. I can't remember the exact procedure, but it was time to 80kts i think. The nav would start the time and call "time" when the calculated time had elapsed. The co-pilot would monitor speed and call "go" at 80kts. If the "time" call came first we would reject. If the "go" call came first then we would continue. A simple acceleration check.

Admittedly this particular procedure had room for confusion, but it could easily be adapted to a modern jet.

[828a]

British Airways [BOAC] were checking the time to 100kts a thousand years ago on their Britania fleet. There was a graph in the flight manual for various weights but it always seemed to be 23 seconds . It was the Captains responsibility to start the stop watch. I've never been able to understand why it was not used on the modern fleets. Perhaps it's just too do. !!!!! 828a

[Notso Fantastic]

Don't like the idea. There is enough to look at and monitor whilst handling an accelerating aeroplane ready to handle abandon take-offs, gusts, engine failures, technical failures on the take off roll. The performance of the aeroplane has already been calculated. All you need to do is ensure you have the thrust you need, light the blue touchpaper and go. The acceleration will take care of itself. Fancy acceleration monitors will just add yet another dangerous, distracting complication. There are so many safety factors built into the performance (ie 50% or 150% wind component). If you start your takeoff roll with a 40kt headwind, you're already well on the way there. The big drawback to me is that there will be a greatly increased incidence of abandoned takeoffs due to 'acceleration problems'- these will inevitably arise at high speeds, and stopping may well be more dangerous than going- an abandoned take-off should not be viewed as a 'safe event'.

[White Knight]

Notso - maybe you've already forgetten MK's recent disaster in Halifax!!! A stopwatch with an approx. time to 100 kts might have helped them realise things weren't right.
Also remember "Palm 90" that crashed in the Potomac! Same thing acceleration wise.

And really it's NOT difficult to glance at the clock at the 100 kt call is it??

[Notso Fantastic]

I am reluctant to discuss the Halifax incident and make any implication the crew were somehow to blame (as you appear to imply) until facts come out. It seems somehow wrong to point a finger at people who cannot defend themselves when we don't know yet what it was about.

I still stand by my judgement that the last thing you need on takeoff is an added distraction. This is from 23 years flying heavy jets on limiting runways. The performance has many safety factors in it. I believe there would be even more accidents in the form of unnecessary overruns with such a system. But I don't expect everyone to agree- just tossing an opinion in to be savaged by the wolves!

[White Knight]

I don't seem to be "implying" that it's the crews fault at all, I just state that that they may have been able to pick up the fact that something wasn't right at an earlier stage.

[csomesense]

Hey guys! An intelectual discussion... not often seen on this site!

I have a graph depicting acceleration versus time on a DC-8.. but the unfortunate thing is that it's all based on someone's old calculations on a -55. As you can imagine, these wont even get close with a -62, a -63, a -73 or anything above that, what can be one for those of us who deal with this in the real world, and don't have enough brain-power to deal with all the added worry of "will my heavy -8 get airborne out of this terribly short strip, or not? (completely disregarding the 747 YHZ site clever inputs!)

As I write this, my collegues advise me to keep away from discussions like this, as they don't concern me...

But I must ask (and I plead with 747Fecal and A-Floor not to respond), am I wrong in the gen I have been given, or should I just carry on as I do, pleading to get these antiquated bits of noise-generators into the air?

[Loose rivets]

Although my previous post in this thread was more than a little based on reminiscences, the subject matter is something I have always felt strongly about. I'll go out on a limb here: I think performance graphs are nothing more than a starting point; the final calculations have to be based on skill attained over time.

A f'instance. I was being checked by my boss on a flight into a small airfield in Northern Yugoslavia. (BAC 1-11) He did the T/O on the return, and somewhere over a rocky beach, I called V1. My concern had been expressed verbally and by fire-walling the power around half way.

The overall performance had been degraded ( I have always assumed ) by the huge uneven concrete slabs thundering away under the wheels, presumably displaced by movement of the landmass. There was nothing in the tables about runways that are shaking the aircraft to pieces.

There were a couple of other times as a F/O, where I have been left stuffing the power through the panel and playing the flaps like some musical instrument. In all cases there was no reaction, or post flight analysis. Bad, bad days, but the graphs had all been ‘correct'.

There has to be a better way. As has been mentioned, all the data are there...the hardware is in place in a lot of cases. Let's have the software.

[828a]

Notso Fantastic;

I can not understand why you or anybody else would consider checking the time to 100kts as being an added work load. BOAC"s system on the Britania was for the F/E to extract the time in seconds from the Flight Manual and write it on the T/O data card which was then placed on the pedestal for all to see. The timing was done by the Captain only so that when the wheels started rolling he started the aircraft clock's sweep hand [ which was adjacent to the ASI ] and at 100kts he simply noted that the sweep hand was near the time . It was a silent operation as there were no calls from other crew members. If you happen to know of anything in aviation that requires less brain power than this then please tell me as I'd like to know about it too. 828a

[6100]

NotSo

You are right, it is added workload, but I know of two instances in my airline where incorrect data was fed into a computer which spat out more incorrect data, none of which was picked up by the crew. This had the result of a very heavy aircraft taking off with a significant derate in place. By the grace of god and the fat built into aircraft certification, both planes flew when they really should not have.

Had these guys had an acceleration check the problem would have been picked up early and would have been nothing more than a low speed reject with safety report to follow, as opposed to a very near major disaster.

So yes a distraction, but one that would probably save lives somewhere one day.

[DOVES]

Dear Sirs.
Perhaps my 19.000 Hrs, 38 years, and 12 different liner of flying gave me the right to insist on my opinion.
The CHECK "Time to 90 kts" (not to 100 as I said in a preceeding post) could have been of some help in so many occasions.
I found, among my many manuals the SE 210 Performance Chart.
I don't know if it's possible to include in this kind of mail an image file (TIF, JPEG, BMP); if yes will somebody please let me know how?
By the way: entering in it with Temp., P.A., A/C Weight, Wind Comp., Rwy Slope, Eng. bleeds, Flaps position, Rwy contamination,
it was possible to calculate the acceleration time (sec.) to 90 kts.
P.S.: At that time it was used to Take Off with the engines at full throttle, but should not be a big deal to introduce in the chart another parameter: the engines derating due to a certain assumed temperature.

[square leg]

Today we took off with 73.0 tonnes. The time to 100 kts was 23 seconds. It was a FLEX T/O.

I thought it was not that difficult to glance at the stopwatch at the "one-hundred" call. My eyes are usually scanning all over the cockpit screens and the runway in various patterns, depending on the actual conditions obviously.

Justathought

[RatherBeFlying]

Those without charts giving time to V1 / 100 kt./ 90 kt. can begin charting the weight, derate etc. against time taken on the stopwatch -- and start checking the airspeed when that time comes up.

Be careful about using the time measured at MTOW with derate when you're light as you will likely be much farther along the runway when that time comes up.

[mcdhu]

Interesting thread!

A low tech facet to this is the fuel flow during t/o. On the minibus, the ff on a max flex t/o at sl should be around 2x3150kg and at Toga shoul be around 2x3600kg. With that, n1, and the speed trend arrow, which should be off the top of the pfd, you have a pretty good idea about how its all going.

Cheers all,
mcdhu

[None]

As Jabbara and Flight Safety discussed, I like the IAS at one of the TDZ markers that are on most runways. In the US, they are the 1,000 ft "fixed distance" markers. For ICAO, it's the "Aiming Point" markers, 400m (1,312 ft).

I wonder how difficult it would be to determine minimum acceleration check speeds for these points based on the selected takeoff performance (thrust and flap settings), and assumed weather and runway conditions?

In theory, all that would need to be achieved is the minimum acceleration. If you reach that speed prior to the check distance markers, you need not pay any further attention to that aspect of the takeoff.

I like that I would be looking at two things I already look at on the takeoff roll; IAS and outside. I also like knowing the progress early in the takeoff roll. This would likely avoid an abort above 100 knots

[FullWings]

I've just read this thread with great interest. The whole TOPM idea is something I've been thinking about ever since I started powered aviation.

I agree with many posters about the 'scatter' in normal aircraft performance and the difficulty in separating something 'wrong' from acceptable variation.

How about this:

The FMC knows the runway, the weight and configuration of the a/c, etc. This can be passed on the the TOPM device, along with airspeed/groundspeed/temperature/pressure, whatever.

During the takeoff roll the TOPM continuously monitors the position and acceleration and gives a warning only if you are not going to get airborne and/or clear the initial obstacles. That level of performance is quite far removed from a 'standard' all-engine departure, so shouldn't lead to many false alerts.

Once the technology has been proven and accepted, we can move on to TOPM II which will warn of much smaller shortfalls/anomalies.

Most glass PFD's have a 'trend vector' on them - as someone else said, it's not exactly rocket science to have an 'expected vector' beside it. The bigger the difference, the greater the problem...

[farqueue]

Can I point everyone to
http://www.casa.gov.au/avreg/fsa/download/00sep/fsa12-15.pdf

Summary: Wasps nested in EPR probe of 737. The nice relaxing start to the take-off got worse in a big way! Includes some discussion of monitoring time/speed to distance.

[john_tullamarine]

These sort of devices have been on the agenda for many decades. If I recall correctly, Grover discussed such a device of his development in his performance book.

FYI, the author of the article mentioned in the previous post is a very active PPRuNe contributor ...

[OverRun]

Updated link to article mentioned by farqueue. The Halifax MK thread and preliminary report has spurred interest in this topic.

http://www.casa.gov.au/fsa/2000/sep/FSA12-15.pdf

[Confabulous]

Hang on - isn't deceleration being checked by computer during braking? Surely all you'd need to do is modify that system to output positive as well as negative accel values to the crew - green/amber/red lights, as well as a trend vector - I believe the Rafale uses an Nx value to monitor takeoff performance instead of slam checks?

[AirRabbit]

White Knight wrote:
Notso - maybe you've already forgetten MK's recent disaster in Halifax!!! A stopwatch with an approx. time to 100 kts might have helped them realise things weren't right.
Also remember "Palm 90" that crashed in the Potomac! Same thing acceleration wise.
And really it's NOT difficult to glance at the clock at the 100 kt call is it??
I completely agree that some kind of “acceleration check timing” should be a rather simple thing to accomplish in the cockpit. However, I think there would be some parameters that should be taken into consideration that will ultimately affect that timing; and I believe that the airplane/engine manufacturers would be able to supply that sort of data within minutes if it were to be required by the regulatory authority or if it were profitable for those manufacturers to do so. Further, I believe such timing information would be applicable with the older "steam gauge" airplanes as well as the modern FMS equipped and/or electric airplanes.

And, most respectfully, once again, I would point out that the “Palm 90” accident in Washington, DC, while there was, indeed, an acceleration issue (the crew very likely set takeoff EPR with the PT2 probes blocked, yielding an inaccurate reading) it was not the acceleration (or lack thereof) that caused that particular accident.
________
AirRabbit

[sky9]

The problem with stopwatch and acceleration is that it doesn’t tell you if you are on the wrong runway or started from the wrong position on the right runway. GPS is now sufficiently accurate for a computer to know what runway the aircraft is on and the distance to the end on the tarmac.
I could see no problem with having a predicted ASI and an actual ASI tape, with the actual laid over the predicted so a warning would only show if the acceleration was inadequate.
With a bit of refinement it could also be used for landing.

[AirRabbit]

Surely, with the technology that exists in today’s environment, there is bound to be a way by which the information being discussed here could be made available to the flight crew. Notwithstanding the accuracy of the many varied GPS receiver units out there today (and the very accurate ones are considerably more costly), there would remain a requirement to upload all of the then-present, pertinent information – things like runway slope, head/cross/tail winds, runway contaminants, etc. – all of which affect acceleration. Additionally, the system would necessarily have to take into consideration gross weight, configuration, and engine performance values – the resulting performance for which might be best supported by the airplane and engine manufacturers. My opinion is that this would present an additional burden (read, cost) to the airline. Finally, display of the results of the calculations (even along the lines of a “predicted” vs. “actual” acceleration tape or graph, as suggested by sky9) probably would not be terribly difficult in many of the modern cockpit displays, but would, I think, require some substantial retrofit for all of the earlier machines.

As I said earlier, the motivation to incorporate (let alone retrofit) such systems could be accomplished by direct regulatory requirements – but would likely incur at least some (and more likely) substantial resistance from those who would have to bear the cost. The argument would likely be, “The statistics just don’t show that this costly system would prevent any of the deaths recently occurring in aviation accidents. Why punish (insert name of objector) by requiring this additional, costly system, into an already overly busy cockpit?” The system, at least in the US, would have the regulatory authority defending their position to a congress (representing the more influential financiers of their on-going political careers) demanding to know why they should back such a costly and potentially impotent system. The bottom line would be the accuracy and viability of such an acceleration measurement system and who would bear the burden of liability in the event of a malfunction of the system or an incorrect operation or interpretation of the system display.

Solve those problems to the satisfaction of everyone involved (pilot, airline, airplane manufacturer, engine manufacturer, regulator, government, and public) and you’ll probably have a useful system. Otherwise, who knows?

My personal belief is that the best opportunity to get such a system into the cockpit, would be for the airplane and engine manufacturers to develop a relatively simple, low cost option that could be ordered when the airplane was delivered – preferably one that could be easily and cheaply retrofitted to earlier aircraft. This would take the burden off of the airplane and engine manufacturers (as they would be providing such a system). It would take the burden off of the regulatory authority and its respective government (as there would be such a system available). The liability burden would be placed somewhat on the pilot (to operate and interpret the results correctly and take appropriate action – but the pilot already has a myriad of identical responsibilities in the cockpit now – one more would hardly be prohibitive). I believe that the airline would bear the lion’s share of the responsibility to purchase and/or install such a system. If they did not, the liability incurred by the airline after an incident or accident that could be linked back to their failure to purchase/install such a system is likely to be substantial. This potential may just be enough of an “incentive” for them to “bite the bullet” and appropriate the necessary time and funds to equip the fleet -- before something happens (we would all hope).

_______
AirRabbit

[sky9]

I don't know the cost of a 747 accident, however there have been recent cases of aircraft departing on the wrong runway and aircraft landing late with a tailwind and contaminated runway.
Personally I suspect that such a system would pay for itself within a couple of years.

[alf5071h]

A significant issue with takeoff acceleration checking systems is the nature of their failure. Simple systems – accel / distance / timing may have a failure rate that would increase the number of rejected take offs, which in having their own degree of risk, may increase the overall risk of operation.

More sophisticated systems are also subject to error, notably human mistakes. A computerised acceleration check is of little use if the system depends on an accurate aircraft weight, but the crew miss set the value, as may have happened in Halifax. Of course with additional complexity weight input could be automated, but this and the associated cost has to balance the overall risk in operation.

Most operations depend on appropriate human behaviour, this cannot always be guaranteed, but with crew crosschecking, and simple gross error checking systems (time to 90kts) the risks should be constrained to manageable levels. Then there is ‘experience’ and ‘seat of the pants’ feelings which should not be discounted, but are often absent in today’s operations. Where risks cannot be contained then technology has to be used e.g. GPWS / EGPWS for CFIT.

Overall the problem is still with the human, s/he, we, all make mistakes, but equally humans can be a powerful error detection system – providing that they are fit, not tired, or fatigued, etc, etc.

[Loose rivets]

As mentioned early on in the thread, all those years ago, a retired air-force colonel here in Texas had proposed and patented a system that announced ‘vocally' the distance traveled down the runway. I think it was called "Tell Me." The pilot would have heard one one one one Two two two two, etc. etc.

I immediately realised that if one used the marker receiver, the kit was in place on almost all aircraft...it just needed fan typ aerials at say, one thousand foot spacing--and switched to increase in the correct direction. It was some days later that I realised that this signal could also be encoded with data for a performance computer. Needless to say, it would soon have become obsolete with the ILS DME installations just over the metaphorical horizon.

I always felt that it required a ground-based reference. So my competitors Doppler system (which was frequently dropping out) and indeed subsequent attempts with INS systems, would not have had that very solid reference to the runway that ground transmissions would offer.

I'm surprised that performance computers have not become a reality years ago. I never felt that it would cause too much mental clutter during the acceleration run, indeed watching one's speed increasing ahead of a scheduling indicator, would I fancy, be rather reassuring.

[LGB]

For any aircraft, assuming a constant acceleration, calculating the time to accelerate to a given speed within a certain distance is quite simple. Knowing that the acceleration is really not linear (especially with engine failed after V1), this is just a rule of thumb of course, but it beats purely relying on your sixth sense.

Note: The formula below have to be corrected for the decreased acceleration as speed, and thus drag, increases


Given: Runway length, V2

Example: 3500 meters, 180 knots

(Knots to m/s is a factor 1.944, 180 knots = 93 m/s)

Time to accelerate is

2 x 3500 / 93 = 76 seconds

Average acceleration is
93^2 / (2*3500) = 1.23 m/s^2

Knowing the time to V2 is too late in the take-off roll for the calculation to be useful for a possible reject.

In our company we use a call of "80 knots". If I want to be at V2 of 180 knots by 3500 meters, 80 knots should latest be reached about 34 seconds after brake release (for a linear acceleration). (80 knots is 41 m/s, divided by 1.23 m/s^2)


A formula to calculate time to 80 knots (or whatever speed you want) for any aircraft, assuming constant acceleration is:

(41 x 2 x runway dist) / V2^2

If you are at 180 knots in V2, the following rule applies for all runway distances:

Runway dist/100 = seconds to 80 knots, i.e. 2800 meters = 28 seconds.

A simplified formula (3% off, but a lot easier), using knots and meters directly:

The formula has V2 divided twice, as some simple calculators does not have the square function. You can even do this calculation with your mobile phone calculator! (If you do have a sqr button, divide by V2 only once, but use that button)


(Vtc = Velocity TimeCheck)

Time = Vtc x TORA x 4 / V2 / V2

Example:

Vtc = 80 knots
V2 = 140 knots
TORA = 2400 meter

Time = 80 x 2400 x 4 / 140 / 140= 39 seconds


Prefer feet instead of meters for runway length?

Make it times 1.2 instead of times 4 above

If you use the same speed every time you calculate, multiply that speed by 4 (meters) or 1.2 (feet) once and for all, to make an even more simple formula, like for time check speed of 100 knots:

400 x TORA / V2 / V2 (meters)
120 x TORA / V2 / V2 (feet)

One factor yet to be corrected is the issue with the acceleration being non-linear. If you really reached the time check speed exactly at the calculated time, you probably would not go airborne in time.

Would some of the wise guys here say if this is possible - can a rule of thumb be made that fits most aircraft? Like two thirds of the time calculated above?

I am only after a rule of thumb formula, not a precision calculation, is it possible?

[FearlessFreep]

Just a quick perusal of these posts I see a few things that are not mentioned with any frequency and that is runway slope and friction coefficents. Since there are many other contributing factors to takeoff performance other than the raw factors of meteorlogical conditions and the basic physics of acceleration it sounds that this "time to speed" calculation should be something that is addressed in runway analysis specific to each airport/runway. I would not think that it would be too difficult to implement those calculations into the total results.

Where I work we do not have the benefit of fancy FMS systems. Everything is done the old fashioned way - by hand!

Then there is ‘experience’ and ‘seat of the pants’ feelings which should not be discounted, but are often absent in today’s operations. Where risks cannot be contained then technology has to be used e.g. GPWS / EGPWS for CFIT.

Overall the problem is still with the human, s/he, we, all make mistakes, but equally humans can be a powerful error detection system – providing that they are fit, not tired, or fatigued, etc, etc.

I think that this is an excellent point and one that is not stressed enough. While rolling down the runway I endeavor to check the "acceleration rate" of the airspeed needle and the corresponding position on the runway. It's all just a feel kind of thing. It would be handy to have a timing to back things up with.

It is interesting to see that this acceleration issue was addressed in the past. More times than not we can find solutions to problems taken care of in the past.

In my own operations I will calculate the V speeds for takeoffs in the "Normal" (Reduced Thrust) category and "Max" Thrust. I include in my takeoff breifing these speed differences, that if we need to we will apply "Max" thrust if the takeoff being performed is a "Normal" one and if during the takeoff roll there is insufficent acceleration past V1.

I tend to be conservative in the utilization of "Normal" thrust in conditions that would be detrimental to our takeoff performance but do not negate the use of "Normal" thrust such as low barometric pressure, runway slope, relatively high gross weights.

[Mad (Flt) Scientist]

@LGB

Obviously the underlying physics are relatively simple, and one could derive a rule of thumb in a manner similar to that you describe. But a couple of points:

TORA is pretty much irrelevant as a number for this purpose, since my time to a given speed should be dependnet upon my thrust and weight, not upon how much runway is in front of me. A "required" rather than "available" distance would at least be related to the aircraft (but TODR isn't much more use, because ....)

Similarly, V2 is an in-air speed, and really doesn't help you out much; there's too much going on to vary the acceleration between V1 and V2 for V2 to be much use in figuring out ground accel numbers.

What you'd really want are distances and speeds for a defined, on-ground, point - either at V1 or Vr would be best; then you could put those in your formulae and simplify them if you wanted a simple rule of thumb. The problem, of course, is that OEMs don't routinely publish distances to intermediate speeds, so the data aren't readily available.

To be honest, if we were dealing only with operations at max thrusts it would be relatively trivial to add an axis to a V1 vs weight type AFM chart which also showed time, so that you could read off both V1 and time-to-V1 from the same chart. The problem would come when people flex, as the time will obviously vary. Maybe there could be some kind of factor based on the amount of flex, but now we've added a new item to calculate, which wasn't the idea.

[alf5071h]

FearlessFreep re your excellent point “ check the "acceleration rate" of the airspeed needle and the corresponding position on the runway”, did you mean checking the airspeed trend vector on the modern ASI?

If so, I agree that this is of value, but for reliable results some knowledge of the technicalities behind the display are required, i.e. if the display is only derived from ‘speed’ then the display is relative to the air-mass acceleration - which still is very important to getting airborne; this is not the same acceleration check as a display that uses an ‘inertial accel / speed mix’, although both are valid as a gross error check. Similar use can be made of HUD FPV acceleration cues where displayed.

The issue of reduced thrust is another opportunity for the pilot to make a judgement on the safety margins available; how many of us really consider ‘what if’ to decide not to use the reduced option, i.e. on a wet runway reduced thrust is not used if it's the regulation, but if not mandated, then what?

The failure to provide a technical solution to the acceleration problem (due to many variables and integrity issues) is similar to that of providing an alert before landing if the aircraft’s energy is too high for the runway length / conditions. Even with generous safety margins many overruns still occur; the numbers of events suggest that this is a greater problem than the take off case.

[FearlessFreep]

alf5071h

You bring up some interesting points. Where I am employed we are glass cockpit equipped - only in that there are little pieces of circular glass scattered all over the place. I fly 74 Classics if that gives you any insight.

The acceleration values that I would be interpreting would be I assume specifically based on as you put it airmass - acceleration. I doubt that the CADC's in our aircraft would be inputting any inertial data. I guess you could on the mighty Carousel IVA select TK/GS hit the hold button and watch the acceleration units, though quite frankly there is a bit too much other stuff going on during takeoff in the Classic. What I am looking for, and this might be hard for me to fully elucidate is a rate of change or for lack of a better term "airspeed needle acceleration" that would be commensurate to previously noted values. If at any point I see a notable lag I will quickly check the opposite side ASI to see if it is reading the same. All of this is while monitoring our relative runway position. During the takeoff run I am always endeavoring to constantly monitoring (of course this is tempered with fatigue constraints - I cannot guarantee that I will be at 100% efficiency 100% of the time) our engine indications, acceleration, airspeed and runway position. These are the only tools that I have to utilize at this point in time.

Thinking more of the timing values that people have used in the past I do not know how effective these would truly be. There are so many variables involved that could change the value by a number of seconds which might remove any safety factor that the timed value would represent. Some of these factors are; rolling takeoff or static takeoff, how quickly and efficiently takeoff power is set, engine age and efficiency, i.e., available rated thrust, delays in starting the timing and any of a number of other factors that could either add or subtract a few seconds. There might be a tendency that once people would see that the timing/acceleration values were met an assumption could be made that everything was ok. With this goal met there would be a possible lack of further vigilance during the takeoff run.

As far as selecting reduced over max thrust, standing water or clutter (as per definition) will negate it's use specifically by regulation (at least in our case). I will select max thrust dependent on a number of different variables. Operating out of PANC with low barometric pressure and close to max allowable takeoff weight usually is always enough to raise my eyebrows. I have found by experience that the runway analysis does not appear to truly represent actual performance in those conditions. If you have max thrust available you might as well use it. When you consider that we fly old aircraft with old engines I am going to be conservative in my utilization of reduced thrust under any conditions that I might construe as being a risk to safety.

[Loose rivets]

Reading between the lines, the thread keeps coming back to a need for a real distance achieved. IMHO this should be second by second, not just at some arbitrary point like V1.

I have mentioned in the past, that I have had two occasions where a warning would have been of great benefit. Both occasions I was a young copilot and did not have the authority to abandon the takeoff. I think I mentioned that I thought that it was due to uneven slabs of concrete at the Northern Yugoslavian airfield and later a grossly overloaded aircraft. Both BAC 1-11s

This was a long time ago, and it was only two instances in 40 years, but on both occasions the captain took no action whatsoever. (At the time or later.) Both times I fire-walled the power and sat and waited...with other hand on the flap lever.

Perhaps it's the reason that I took a personal interest in performance computers.

[westhawk]

It would seem to me that a takeoff performance computer function in the FMS could be adapted to read real-time input sensors like wheel speed, GPS, acceleration and ADCs to keep a running prediction of where on the runway the vital performance speeds of V1 and Vr will be achieved. When those points are projected to occur beyond the last points on the runway where either a stop or a go(as applicable) are still possible, a warning is generated. Rate of IAS increase to determine the go point and rate of groundspeed increase for the stop point. Wheel speed and inertial for data validation of GPS position. Might need faster processors.

As to how it may be integrated and how it will be used to make go/no go decisions during the takeoff roll.... Let the games begin!

Just my $0.02

Best regards,

Westhawk