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Why not measure acceleration instead of using derated flat throttle percentage?

Posted: Fri Apr 09, 2021 9:17 pm
by darkpino
Hi all,

Tonight we had a discussion in our friends WhatsApp group after reading this news message:

https://www.theregister.com/2021/04/08/ ... e_mistake/

Imo it’s kind of overdone and a sensational article but that’s not really the point now. On of my friends (which is obviously pretty smart) asked me why they are calculating this and entering these parameters by hand if you can also just enter the runway length for example and let the FMC measure the acceleration rate. I thought it was actually a pretty smart idea and I don’t have the answer why they don’t. So I thought: I’ll just ask here?

Looking forward to the answers,

Martijn

Re: Why not measure acceleration instead of using derated flat throttle percentage?

Posted: Fri Apr 09, 2021 9:48 pm
by GalaxyFlyer
We did in the USAF, acceleration check speed—time to 100 knots as an indication of planned thrust being achieved. One problem—has to be done from a standing start, not rolling.

Re: Why not measure acceleration instead of using derated flat throttle percentage?

Posted: Sat Apr 10, 2021 12:57 am
by tb727
We did it in the Falcon 20, well the original one we had with CF700-2D2 engines. I want to say it was normally 20ish seconds to 100 knots but that's fuzzy. We did standing takeoffs as SOP in them.

Re: Why not measure acceleration instead of using derated flat throttle percentage?

Posted: Sat Apr 10, 2021 2:15 am
by 77west
Does the A350 not have a takeoff safety system that can intervene if it thinks the runway length remaining and rate of acceleration would lead to safety issues?

Here's a video where it miscalculated apparently

http://www.youtube.com/watch?v=MKtPlj93 ... sGuy%7CTPG

Re: Why not measure acceleration instead of using derated flat throttle percentage?

Posted: Sat Apr 10, 2021 3:51 am
by Lukas757
darkpino wrote:
On of my friends (which is obviously pretty smart) asked me why they are calculating this and entering these parameters by hand if you can also just enter the runway length for example and let the FMC measure the acceleration rate.


Simply because its too late to measure the weight at the runway. This would work as a backup, yes.
But you can’t just accelerate down the runway and reject the takeoff every time you find out you’re too heavy.

Re: Why not measure acceleration instead of using derated flat throttle percentage?

Posted: Sat Apr 10, 2021 4:36 am
by Starlionblue
As Lukas757 says, it's too late at the runway. Even if you used an acceleration measuring system, you have to work out the performance parameters ahead of time anyway. It's all done before engine start, because you need to know what reduced thrust you enter, if you can do an intersection take-off, if you can meet the departure requirements, and so on.

Side note: We rarely used a fixed derate. Typically only on a contaminated runway. Flex/assumed temp is normally used.


77west wrote:
Does the A350 not have a takeoff safety system that can intervene if it thinks the runway length remaining and rate of acceleration would lead to safety issues?

Here's a video where it miscalculated apparently

http://www.youtube.com/watch?v=MKtPlj93 ... sGuy%7CTPG


To the best of my knowledge, the A350 cannot "intervene". The article linked in the video description states QR reps said the plane "aborted on its own". I've never heard of such a mechanism on any aircraft, but I suppose it may exist. You might get a rather stern warning but even the configuration warning won't stop you from trying to go.

Further down in the article, another QR rep is reported as saying an "indicator" was responsible. That's a bit vague. I would interpret it as the pilots getting a warning or caution and then rejecting. Airbus SOP states you should reject for any ECAM when below 100kn. Once above 100kn, most ECAMs are inhibited, and you would reject only for engine failure, engine fire or if there were serious doubts with regards to the ability of the aircraft to fly. It's a bit hard to tell how fast the aircraft was going when the take-off was rejected.

What makes you think there was miscalculation involved in that QR take-off? There are plenty of reasons to reject a takeoff, and they are not uncommon events. Miscalculation is not typically a reason. Unless you feel a very definite lack of acceleration, you trust the parameters are correct.


Of note is that the A350 has a ROW/ROP system. Runway overrun warning gives warnings on approach when the entered landing runway is too short for the conditions, and runway overrun protection screams during the rollout at you if the aircraft senses the aircraft won't stop before the end. This CAN automatically apply max braking, but it is only active on landing.

Re: Why not measure acceleration instead of using derated flat throttle percentage?

Posted: Sat Apr 10, 2021 6:48 pm
by LCDFlight
Starlionblue wrote:
As Lukas757 says, it's too late at the runway. Even if you used an acceleration measuring system, you have to work out the performance parameters ahead of time anyway. It's all done before engine start, because you need to know what reduced thrust you enter, if you can do an intersection take-off, if you can meet the departure requirements, and so on.

Side note: We rarely used a fixed derate. Typically only on a contaminated runway. Flex/assumed temp is normally used.


77west wrote:
Does the A350 not have a takeoff safety system that can intervene if it thinks the runway length remaining and rate of acceleration would lead to safety issues?

Here's a video where it miscalculated apparently

http://www.youtube.com/watch?v=MKtPlj93 ... sGuy%7CTPG


To the best of my knowledge, the A350 cannot "intervene". The article linked in the video description states QR reps said the plane "aborted on its own". I've never heard of such a mechanism on any aircraft, but I suppose it may exist. You might get a rather stern warning but even the configuration warning won't stop you from trying to go.

Further down in the article, another QR rep is reported as saying an "indicator" was responsible. That's a bit vague. I would interpret it as the pilots getting a warning or caution and then rejecting. Airbus SOP states you should reject for any ECAM when below 100kn. Once above 100kn, most ECAMs are inhibited, and you would reject only for engine failure, engine fire or if there were serious doubts with regards to the ability of the aircraft to fly. It's a bit hard to tell how fast the aircraft was going when the take-off was rejected.

What makes you think there was miscalculation involved in that QR take-off? There are plenty of reasons to reject a takeoff, and they are not uncommon events. Miscalculation is not typically a reason. Unless you feel a very definite lack of acceleration, you trust the parameters are correct.


Of note is that the A350 has a ROW/ROP system. Runway overrun warning gives warnings on approach when the entered landing runway is too short for the conditions, and runway overrun protection screams during the rollout at you if the aircraft senses the aircraft won't stop before the end. This CAN automatically apply max braking, but it is only active on landing.


OK, but it would be pretty easy to fit an accelerometer to prove that, within 10% margin or something, you are getting the performance you expected. I am thinking it could detect load input errors as a last resort, and throw up a warning to abort at 50-80 knots while it is still very low risk to do so. But IANAP

Re: Why not measure acceleration instead of using derated flat throttle percentage?

Posted: Sun Apr 11, 2021 1:04 am
by Starlionblue
LCDFlight wrote:
Starlionblue wrote:
As Lukas757 says, it's too late at the runway. Even if you used an acceleration measuring system, you have to work out the performance parameters ahead of time anyway. It's all done before engine start, because you need to know what reduced thrust you enter, if you can do an intersection take-off, if you can meet the departure requirements, and so on.

Side note: We rarely used a fixed derate. Typically only on a contaminated runway. Flex/assumed temp is normally used.


77west wrote:
Does the A350 not have a takeoff safety system that can intervene if it thinks the runway length remaining and rate of acceleration would lead to safety issues?

Here's a video where it miscalculated apparently

http://www.youtube.com/watch?v=MKtPlj93 ... sGuy%7CTPG


To the best of my knowledge, the A350 cannot "intervene". The article linked in the video description states QR reps said the plane "aborted on its own". I've never heard of such a mechanism on any aircraft, but I suppose it may exist. You might get a rather stern warning but even the configuration warning won't stop you from trying to go.

Further down in the article, another QR rep is reported as saying an "indicator" was responsible. That's a bit vague. I would interpret it as the pilots getting a warning or caution and then rejecting. Airbus SOP states you should reject for any ECAM when below 100kn. Once above 100kn, most ECAMs are inhibited, and you would reject only for engine failure, engine fire or if there were serious doubts with regards to the ability of the aircraft to fly. It's a bit hard to tell how fast the aircraft was going when the take-off was rejected.

What makes you think there was miscalculation involved in that QR take-off? There are plenty of reasons to reject a takeoff, and they are not uncommon events. Miscalculation is not typically a reason. Unless you feel a very definite lack of acceleration, you trust the parameters are correct.


Of note is that the A350 has a ROW/ROP system. Runway overrun warning gives warnings on approach when the entered landing runway is too short for the conditions, and runway overrun protection screams during the rollout at you if the aircraft senses the aircraft won't stop before the end. This CAN automatically apply max braking, but it is only active on landing.


OK, but it would be pretty easy to fit an accelerometer to prove that, within 10% margin or something, you are getting the performance you expected. I am thinking it could detect load input errors as a last resort, and throw up a warning to abort at 50-80 knots while it is still very low risk to do so. But IANAP


The idea of accelerometers has merit, but I think you might get a lot of spurious warnings. Shifting winds can change acceleration, for example.

If you've put in the correct figures, the acceleration will be correct. If an engine doesn't provide the required thrust, the engine instruments will tell you.

Runway overruns on takeoff don't occur because the aircraft mysteriously doesn't accelerate at the required rate. Causes include using the wrong intersection, calculating based on or entering incorrect figures, not handling an engine failure incorrectly, and so on.

There is some logic in the FM to detect large input errors. However, the FM doesn't know anything more than the figures we input. The aircraft doesn't weigh itself.


Side note: "Abort" is typically a military term. I've never seen it used in civilian aviation. We "reject" a take-off. And if we want to go around after touch-down, that's a "rejected landing".

Re: Why not measure acceleration instead of using derated flat throttle percentage?

Posted: Sun Apr 11, 2021 1:18 am
by GalaxyFlyer
Funny, we used “reject” in the USAF, at least, in AMC, the command formerly known as MAC.

Re: Why not measure acceleration instead of using derated flat throttle percentage?

Posted: Sun Apr 11, 2021 1:26 am
by Starlionblue
GalaxyFlyer wrote:
Funny, we used “reject” in the USAF, at least, in AMC, the command formerly known as MAC.


So who actually uses "abort"? :biggrin:

:?: :?: :?:

Re: Why not measure acceleration instead of using derated flat throttle percentage?

Posted: Sun Apr 11, 2021 1:41 pm
by GalaxyFlyer
My corporate had a chief pilot who thought he was a military and imposed “abort”, couldn’t get his protégés to change it. I always thought “GO” and “STOP” were better. AMC did use “GO”.

Re: Why not measure acceleration instead of using derated flat throttle percentage?

Posted: Sun Apr 11, 2021 1:47 pm
by RetiredWeasel
Starlionblue wrote:
GalaxyFlyer wrote:
Funny, we used “reject” in the USAF, at least, in AMC, the command formerly known as MAC.


So who actually uses "abort"? :biggrin:

:?: :?: :?:

Unless things have changed in the last 15 years, fighter aircraft use the term "abort" in their boldface T/O abort procedures. Abort boldface (memory item) for F-4:

1. Throttles idle
2. Chute-deploy
3. Hook-down
4. Brakes-apply.

Re: Why not measure acceleration instead of using derated flat throttle percentage?

Posted: Sun Apr 11, 2021 1:50 pm
by e38
GalaxyFlyer wrote:
we used “reject” in the USAF, at least, in AMC


"Abort" is standard terminology in the USAF AMC KC-135 fleet.

e38

Re: Why not measure acceleration instead of using derated flat throttle percentage?

Posted: Sun Apr 11, 2021 2:52 pm
by GalaxyFlyer
More curious, then, but the T-tails and Tankers never merged really.

Re: Why not measure acceleration instead of using derated flat throttle percentage?

Posted: Mon Apr 12, 2021 12:54 pm
by Lukas757
FBW Airbus do compute the gross weight, although only after the aircraft is in the air. If the difference between the entered GW in the FMS and the computed GW is higher than 7 tonnes, an amber ECAM alert „CHECK GW“ will show in the cockpit. This is calculated via the flight computers using data from AOA probes etc.
Obviously this won’t help to prevent an overrun during takeoff.

Re: Why not measure acceleration instead of using derated flat throttle percentage?

Posted: Mon Apr 12, 2021 9:15 pm
by KingOrGod
Starlionblue wrote:
GalaxyFlyer wrote:
Funny, we used “reject” in the USAF, at least, in AMC, the command formerly known as MAC.


So who actually uses "abort"? :biggrin:

:?: :?: :?:


A colleague once told an arriving aircraft that there was "a take off abortion on the runway"... :boggled:

It was just a RTO who was unsure if they could vacate...