I met someone who says he helped develop TCAS, and that it decides which one to climb or descend based on the registration number.
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.
> 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":
Good answers in sibling comments, but for those who don't want to grovel through a lot of stackexchange answers here is the TLDR: There are two reasons:
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.
I don't think the GP was asking about changing heading instead of altitude; I think they were asking about changing heading in addition to altitude. Sure, the altitude change is going to achieve better separation faster, but a heading change in addition to the altitude change will act as a backup in case for some reason something goes wrong with the altitude change.
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.
ADS-B is not guaranteed to be available from all aircraft in most of the airspace below 10K feet MSL. (Basically, if a transponder used to be optional before, ADS-B out is also optional.) Mode-C (altitude encoded transponder replies) are also not required, but the equipage rate is very high at this point, which is less the case for ADS-B among GA aircraft.
I have to imagine that if there was a #3 on that list it would be that introducing a random heading change adds follow-on complexity that could be dangerous itself. Now the planes are not just at different altitudes, but pointed who-knows-where.
You could imagine if, in addition to obeying the altitude change, every plane began to turn rapidly to the right, it could only help the situation? As long as we all agree to turn right in case of a collision warning?
MerelyMortal|2 years ago
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.
ceejayoz|2 years ago
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":
lisper|2 years ago
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.
pdonis|2 years ago
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.
sokoloff|2 years ago
https://www.aopa.org/go-fly/aircraft-and-ownership/ads-b/whe...
rootusrootus|2 years ago
ceejayoz|2 years ago
sgustard|2 years ago