Using the Chrono to measure take off acceleration
 

Hi All,
While some manufacturers are offering undoubtably expensive technical solutions to measuring one’s take off acceleration I have heard from two sources that many pilots hit the stop watch at the start of the take off roll. The suggestion is any performance A aeroplane should reach 80kts in 20 seconds or 100kts in 30 seconds. I have experimented with this on the 320 and a little bit on the 380 and it seems to work however it can depend on exactly when the stopwatch is started. A stationary start is easy, release brakes, set 30%, start the chrono then set thrust. A rolling take off though will potentially have the jet doing a fair few knots when the stopwatch is started.
One source I heard the idea from suggested he rejected a take off because the jet had not reached 80kts by 20s, chief pilot went mad, Commander insisted the jet was weighed and it was found to be ten tonnes over the loadsheet weight.
The performance margins of the new twins (like 787) are so slim, I wonder whether this would also work on them. Presumably they still have to meet the perf A requirements but seem to use up far more runway than other comparable types.
Anyhow long story short, does anyone routinely use this, precisely how do you use it and is there any online guidance?
kind regards,
Leslie

A320 (don’t know for other aircrafts) but written nowhere in the official manuals, thus no one would be ever accept a rejected take off due to this reason.

Trying to lump things into a simplistic "so many seconds to a specific speed" probably is not a great deal of value for the wide range of jet performance seen these days.

The military places some reliance on runway distance boards for the concerns suggested in the OP's post. However, unless you have some objective data it gets a bit hard to come up with homegrown techniques which hold much water.

Having said that, I see no problem, in the usual absence of such data in civil manuals, with a pilot's developing some data for his/her Type by measured observations on the line. One is not trying to obtain anything like precise data, rather gross, indicative data which should alert the thinking pilot to significant acceleration problems in the earlier parts of the takeoff. If one constrains any resulting accel-stop decision to below, say 80-100 kts for the typical jet, then there is sensible purpose to the exercise.

Having been involved in performance measurement and scheduling in previous lives, a sensible approach to the problem should easily yield reasonably accurate data which can be expanded into a useful aide memoire.

I recall reading/hearing somewhere that we should get to 80 by about 1000ft down the runway. That rule of thumb seems to work most days.

That would be good if I had 1,000ft marked from where I started my take off roll by. Thanks for the responses. This would be a gross error check but would have worked as a final layer of defence on the Emirates 340 in Melbourne and multiple other events. Humans have a poor sense of acceleration assessment. Airbus are fitting a monitor system on 350 currently that will flag up insufficient acceleration between 30-90kts.

I used it routinely to compare A320 performance to that of sports car. 0 to 110kt (0 to 200km/h)
It takes around 20-25 seconds tops.
So your figures are rather prudent.

But they're not precise enough to constitute basis for any serious decision. Imho, it doesn't go further than the fun of trying to be quicker than a sports car.

We start the stopwatch as the power goes on but only because of the 5 min limit on T/O power.

For lighter a/c,rule of `thumb`,,a minimum of 71% VR @50% Runway is OK,anything less-Abort..
Otherwise use Newton`s Laws of Motion....

In those aircraft with speed acceleration (trend) it should be possible to develop a useful rule of thumb.
e.g. the speed trend can be matched to an acceleration; a useful value may be 3kts / sec ( 1/3 scale ); with experience - developing the rule of thumb, variations for conditions, wt, etc, can be applied.

FFS. What about the day with a 40 knot headwind?

Hg,start the clock early...

FWIW Airbus TRI Training was big on starting the chrono at exactly 50% N1.

It’s been a long while, but in the C-5 where we did lots check speeds on standing take-offs, 100 knots was about 35 seconds into the roll, plus or minus for extremes. We had a standard of not more than 3 knots below 100 at the end of the scheduled time. Manual had a graph to figure the more accurate number. A friend used to say 100 knots at 3000’, on task-off or landing, you should be at 100 knots after 3000’ or no more than 100 knots with 3,000’ remaining. Funny, the one take-off I remember well, it was apparent to me in the jump seat that it was going to be close. Sure enough, 97 knots at the time and it might 96.5. It’s amazing how much distance goes by accelerating above 120-130 knots.

Haven´t timed takeoff acceleration since my DC-8 days, which ended early nineties. 30 seconds to 100 kts (in the JT3D powered versions). On a hot (and/or) high takeoff that could go up to about 33 second IIRC with the runway end rapidly approaching.

B-52 has a timed take off, timing starts at 70 knots and ends at V1, time extracted from the charts. If you hit the time limit prior to V1, abort.

A-4 used a speed check at a preselected distance marker.

Why not just trust your performance app? If there is something seriously wrong with your aircraft during takeoff, you are going to know about it.
I’ve never heard of this timing acceleration business! That’s what the test pilots do!

Well… a lot of people have punched the wrong numbers into a performance app. Garbage in, garbage out…

80 kt in 20 s is an average acceleration of 2.06 m/s2 or 0.21 g (with no wind) and the distance rolled ~= 1350 ft.
With the same constant acceleration of 0.21g, 150 kt would be attained in 37.5 s for a rolled distance of 4750 ft or 1450 m (still with no wind).
This 20 s limit looks a bit too conservative.

Pretty much all the performance-related incidents and accidents I’ve seen have been due to getting the calculations wrong. This a good place to apply rigour if you’re going to apply it anywhere.

Given that most civil operators use software that minimises engine wear in exchange for runway used, and take advantage of ATM and fixed derates, it would be hard to discriminate by timing between a correct thrust setting and not. If you are achieving the right N1 you are almost certainly getting the thrust you want - the secret is to make sure the the numbers makes sense: MTOW and 70%N1 probably won’t cut it.

I suspect on most types there is too much of a spread between the time to a certain speed on a short runway (or intersection departure) and a long troll down 4,000m with a low flap setting and an overspeed takeoff to have a figure in mind on a stopwatch. There’s a lot more latitude to reject at low speed (<80kts) if you don’t like what you’re feeling but you have to be a bit more rule based at the higher speed end.