Anyone know why the TCAS only instructs the two approaching aircraft to change altitude only, but not heading?
I'm guessing that it does not issue turn instructions because it only gives a Resolution Advisory when a collision is imminent (otherwise they would be giving RAs all the time). At altitude, where the air is thinner, it is much easier for a plane to immediately climb or descend, planes turn very slowly when the air is thin.
https://aviation.stackexchange.com/questions/92495/how-does-...
> But what if the other aircraft is "thinking" the same thing? That's the heart of this question. The answer is, whichever TCAS makes a decision first, and then in case of a tie, the lower Mode S address "wins":
EDIT: FAA PDF on TCAS 7.1 - https://www.faa.gov/documentlibrary/media/advisory_circular/...
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https://www.amazon.com/Black-Box-Thinking-Psychology-Paperba...
1. When TCAS was originally developed, the direction information to the other aircraft was not very accurate, being obtained only via the TCAS antenna. Nowadays GPS information is transmitted via ADS-B and so it's much more accurate, but taking advantage of this would require a major redesign. It may happen eventually, but...
2. Aircraft can change altitude faster than they can change heading. Also, most aircraft can change pitch faster than they can roll, and they are longer and wider than they are high. So effecting enough altitude change to avoid a collision can be done faster than effecting enough heading change to do so.
https://www.aopa.org/go-fly/aircraft-and-ownership/ads-b/whe...
Sure, but so what? The lack of universality is not an impediment to implementing a new TCAS system that uses ADS-B data when it's available. (And BTW, the places where ADS-B is not required are generally places where traffic is sparse and so the risk of a collision is pretty low to begin with.)
Given that the clear priority is now to follow a TCAS RA (resolution advisory) over an ATC instruction, I think that the current TCAS approach is good enough to cause aircraft to miss each other and that a GPS-based redesign is unlikely.
In this particular accident, it seems like commanding a heading change (to the right for the Tupolev and to the left for the DHL flight) in addition to an altitude change would have avoided the collision.
Yes, intuitively being able to change headings seems like a good thing except that 1) it adds expense and complexity, and thus produces new potential points of failure and 2) it's not necessary. The problem was not that an altitude change was insufficient, the problem was that two sources of command authority (TCAS and ATC) contradicted each other. The solution is simply to add a new rule: if TCAS and ATC disagree, go with TCAS.
I never said we must do X. It's a suggestion which might or might not be worth considering. From the article it appears that the TCAS system was designed with the assumption that pilots would always immediately do what it said. This accident obviously violated that assumption. The question is how best to respond to that as far as the design of TCAS is concerned; the eventual tradeoffs might lead to the answer "do nothing to TCAS" but that doesn't mean it isn't worth considering possible changes.
> it adds expense and complexity
Yes, this is always a tradeoff with safety systems. But the heading change rule would be even simpler than the altitude change rule (which requires negotiation of possible conflicts between the TCAS systems in the two aircraft): the heading rule is simply "turn away from the other aircraft", i.e., if the other aircraft is to your right, turn left, if it's to your left, turn right. That seems like logic that could be implemented, checked, and verified fairly easily, if it were decided to do so.
> it's not necessary
When it comes to safety systems, "necessary" is always a judgment call. Yes, cases in which adding a heading change to TCAS instructions would be helpful are rare (like this crash was). But all aircraft accidents are rare. No rule gets followed exactly as it's given 100% of the time. Taking that into account when deciding on what safety systems to implement is perfectly justified.
> The solution is simply to add a new rule: if TCAS and ATC disagree, go with TCAS.
Or more precisely, to explicitly make that a rule worldwide, instead of just in the US as it was before this accident.
Yes, this is the response the system settled on, but this, too, involves a tradeoff: this rule has to be implemented by humans, and humans are less reliable than automated systems. The international air safety system evidently decided that humans were reliable enough in this case for this rule to be sufficient; but that doesn't mean they didn't make a tradeoff.
Heading and altitude are NOT independent. When an aircraft makes a turn, it banks (it's not like a flat turn you make in your car), and inevitably loses some speed because of extra drag, and some altitude because of loss of lift.
It can be compensated by increasing the engine power, but then you really need to predict that as well.
Doing both reeks to me of a solution in search of a problem that’s going to end up the target of some later NTSB report when you hit some weird edge case where the dynamics and dimensions of the two planes aren’t fully accounted for.
Yes, but it's also increasing horizontal separation (from a predicted value of zero), which might still be a reasonable tradeoff to consider.
Yes, I know how planes make turns. It is valid to point out that a TCAS instruction that included a heading change would, at least for the plane whose TCAS instruction was to increase altitude, also have to include an engine power increase, which might not be feasible depending on the altitude (cruise power might already be pretty close to max power). Or else make the turn a very slow one, which would decrease its usefulness in collision avoidance.