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Comment by londons_explore | Hacker News Reader

There is another bug here...

If the plane was told how much weight was onboard, and it knows what throttle it was at... then it should know what acceleration it was expecting.

If it accelerated slower than expected, then there must be something wrong! So abort the takeoff...

Planes should have alerts for this kind of thing. If, 1 second after throttle is applied, the plane hasn't achieved the predicted speed, then the takeoff should be aborted - by this stage, you'll be well under the maximum abort speed, so you can stay safely on the ground and figure out whats wrong. Being overweight is just one possible cause - it might instead be that an engine isn't producing sufficient thrust, and that would probably doom the plane.

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Given that both of these planes took-off well within the acceptable safety margin, I’m not sure you can say that the planes deviated from their expected acceleration profiles enough for a model to confidently say there was an error significant enough to justify an abort.

Ultimately the issue was noticed not because a single plane suffered a failure (one that’s expected to occur occasionally, and where multiple mitigations exist to ensure the failures are safe), but because two planes one after the other suffered an identical failure, a failure that’s only expected to occur rarely.

So ultimately the unexpected event was two closely spaced expected failures. Not the expected failures themselves, which kinda indicates that aborting to prevent the expected failures would have quite a high false positive rate. Aborts and alerts with high false positive rates are themselves safety issues which need to be avoided, so a trade off needs to be made. The fact that the individual tail strikes themselves were completely safe, if not ideal, and the larger issue was rapidly recognised and corrected, strongly indicates the right trade offs have been made.

FWIW, it takes way more than 1 second for the engines to spool up. And an engine not producing sufficient thrust would not doom the plane -- as long as the airplane makes it to V1 (decision speed), it should be able to fly with a single engine failure after that point. And the pilots should already have talked through the takeoff and come up with a go / no-go based on distance: "if we're after this point and we haven't reached V1, we'll abort".

True, but the spool up rate and thrust produced during spool up is all predictable.

Basically, I'm saying the plane should have an internal mathematical model of what it expects to happen, and anytime that sufficiently differs from reality, it should trigger an abort - especially if you're still on the ground at the time!

Hopefully you mean tell the pilots to abort and not start the process itself. What if there's something on the runway or the runway is too short or slick for the plane to decelerate? There's differences between conditions and runways that would have to be accounted for and then you'd have to add a lot more capabilities to the plane's sensors for every other possible issue.

It's often a judgement call when things don't happen quite right and I don't think our technology is anywhere near ready to hand the abort process over to the plane's programming.

This parameter is displayed on some aircraft, notably French (Mirage 2000 & Rafale) which display the longitudinal acceleration (labelled Jx) in the headup display. Part of the takeoff procedure for the pilot is to confirm the Jx is nominal with regards to the takeoff parameters (loadout, density altitude, etc.).

One detail: V1 is the last-chance speed for being able to abort the takeoff within the remaining runway length, but it is predicated on the weight, among other things, being correct. If the airplane is heavier than the figure used in calculating it, you will not be able to stop in time from V1 (you will reach it later and further down the runway, yet need more distance to stop.)

Having said that, a speed-at-distance rule like the one you give here, especially if it was evaluated at a distance before you expect to reach V1, would be perfectly good. There is a rule-of-thumb for small-plane pilots saying you should have reached 80% of the rotation speed before you are halfway down the runway (though that speed is also weight-dependent - as demonstrated in these incidents.) There is a tacit assumption here that your deceleration after choosing to abort will exceed your acceleration up to that point.

No that simple ; what is wind speed and direction for instance, which changes acceleration? Air density will also affect, although that's easier to account for.

These things all have to be taken into account in the original calculation for how much power to use. It is impossible to determine the answer to that question without having the information necessary to calculate an expected acceleration profile for the takeoff.

If you are using approximations in that calculation (such as the average weight of a passenger), then that same approximation can be used in calculating the acceleration profile, and if you are falling significantly short, then it shows that the calculation of the power setting needed for a safe takeoff is unsound in some way. That alone is sufficient to justify an abort.

Can't the plane measure wind speed and direction and air density and all, and account for those?

Not especially well, especially close to the ground.

Some recent Airbus aircraft do have some take-off performance monitoring, though it's not extensive. If the expected acceleration differs enough from the actual acceleration of the aircraft, you'll get a "T.O. ACCELERATION DEGRADED" ECAM message and should reject the take-off.

I have no idea whether Boeing has the same. It's an emerging area, sometimes called a Take Off Performance Monitoring System.

Let’s go back even further: plane is loaded up, fueled and doors are closed. Why is the plane not capable of weighing itself? It seems entirely reasonable, even in an environment where weight savings are important, for the plane to sport the capability to have weight sensing mechanisms on its landing gear to get actual weight.

The 747-400 was one of the first aircraft with that capability. AIUI, customers hated it because it was fiddly.

I have often wondered that myself, and I looked into it a little bit, finding that there have been ideas for actually weighing airplanes on the ramp going back to the 1930s, but nothing seems to have come of it.

One possible reason might be a usually well-justified concern over adding complexity which could, if it fails, be worse than the problem it solves - but what would it take to measure the load on each undercarriage leg? something to measure its extension? Strain gauges on the structure the legs are attached to? I would really appreciate the input from an aero structures engineer here (paging Walter Bright...)

One other issue this would help is with is ensuring that the center of gravity is within limits (after the crash of Air Midwest Flight 5481, where this was a significant cause, the rule for estimating passenger weights was revised.)

On the other hand, an acceleration test will detect a lot of other problems beyond being overweight. There was a Citation crash near Hartford CT last year where it is suspected that the parking brake did not fully disengage. Why not have both, if they are feasible?

It's difficult because any minute amount of wind will affect the final results, and you don't have the full wind profile over the entire aircraft to compensate for it. Easier and safer to track what's on board and calculate the a/c performance based on that + margins.

I take your point; I have no idea of the magnitude of this effect in proportion to the total weight. I do wonder if a combination of the airspeed instrument readings, their fluctuation, and fluctuations in the measured weight could be used to estimate the influence of the wind (an application for machine learning?) but even so, that would mean that it would no longer be a simple solution.

What I’m talking about is just a way to provide something besides a pilot’s hunch to compare against weight data from flight crews. If it was so windy that it would catastrophically throw off the comparison, I doubt they’d even let the planes take off.

That's not how it works. Airliners can safely take off into strong headwinds. A wind blowing across the wings will produce significant lift. In theory it might be possible to compensate for that effect, but in practice it's just too much hassle.

I wondered the same thing. Most metros perform load weighing to adjust the effort. Why can’t the same tech be applied to planes? It may not capture all the data needed, but it wouldn’t be off by 20 thousand pounds and it could be incorporated into a feedback loop, to increase effort if the plane isn’t getting the expected lift.

I think the short answer to this is that because of the forces involved on the landing gear, the load cells would need to be recalibrated more often than would economically be feasible.

Pilots are meant to have a rule of thumb where they set a stopwatch and whatever takeoff power they are using they should reach 80kts within a certain time period, can’t remember exactly but I think it’s about 30 seconds. If not they should reject takeoff before they reach the ‘high speed realm’. Possibly the newest Airbus A350 might have what you describe already, but they don’t tend to back port this stuff to the older systems in older airframe families.

> Possibly the newest Airbus A350 might have what you describe already, but they don’t tend to back port this stuff to the older systems in older airframe families.

It's not that they don't tend to, it's that airlines don't want this — training management and recertification on behavioral changes is costly for them. See Southwest influencing 737 MAX development—MCAS was a result to try to reduce the characteristic changes between aircraft & engine variants. (Band-aid driven development!)

I sincerely hope that the 737 MAX incidents change the industry mentality to encourage safety features to be backported to older airframes, even if it seems untenable now. For example, the 737 MAX uses older crew alerting methods—annunciator lights—not EICAS, which can be thought of as a centralized actionable list of anomalies.

MCAS was used to make the aircraft conform to the regulations, it could not be omitted. It is intended to increase the control force at a point in order to assure control forces steadily increase into a stall. (See 14 CFR 25.203 -- Stall characteristics. (FAR 25.203))

The fact that it was not communicated in the training and manuals was a result of lobbying by the airlines.

junk data in, junk data out — the issue was the weight calculation, so in this case the plane would (incorrectly) think it's performing nominally.

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