In view of the recent tragedy at Reagan National, I’d like to understand with factual responses:
a) Do commercial aircraft use their radar all the time, or only certain times? And, I believe I once heard it is not used below x feet elevation. How far out does the beam detect? In short, could the plane have detected the helicopter in its airspace? And, what was the approximate elevation of the aircrafts at the point of collision?
b) Now, to flip the question based on public information… (If it cannot be answered, I understand). Couldn’t the helicopter detect the airplane?
Commercial aircraft do not have radar in the sense that you are thinking of. They only have weather radar. I can’t speak comprehensively to what military aircraft have but it’s my understanding that those radar systems are for target acquisition and not broad range traffic management.
Nowadays though commercial aircraft have TCAS which is a bit radar-like, but consists of each participating aircraft broadcasting its position in real time, and the receiving aircraft using that info to create a “map” of the surrounding airspace. That said, TCAS does not work at low altitude, and military aircraft may not participate in TCAS at all.
Adding that ATC radar uses aircraft-sent radio transponder codes/signals to detect positions.
They still depend heavily on traditional primary and secondary radar for location and ID. It’s not a purely ADSB system yet.
Why would the transponder not work at low altitude? I can see if the issue is line-of-sight transmissions and over the horizon, but it seems to me the thing should still work at any altitude. IIRC from the map on passenger aircraft (which I assume feeds off the same data) they are quite capable of detecting lower altitudes including airport elevation (ground level).
The transponder continues to work fine at low altitude. Rather, the TCAS system becomes progressively less able to resolve issues as altitude decreases. Largely because the TCAS system can only resolve proximity issues by issuing climb and descend commands to resolve the incident. At 1,000 above the ground, the system is unable to issue any further commands because it can’t instruct one of the aircraft in conflict to descend.
Radio altimeter data is used to recognise when TCAS is too close to the ground to issue RAs. Aircraft operators and flight crews are reminded that:
- TCAS II design progressively inhibits Resolution Advisories (RAs) depending on the height Above Ground Level (AGL) provided by the radio altimeter as follows:
- “Increase Descent” RAs are inhibited below 1,550 ft AGL (± 100 ft)
- “Descend” RAs are inhibited below 1,100 ft AGL (± 100 ft)
- All RAs are inhibited below 1,000 ft AGL (± 100 ft).
Another reason to disable TCAS at low altitude is that when you are approaching an airport, guess what’s there? Answer: A whole bunch of airplanes on the ground right near where you are going all of which are broadcasting their altitude.
Absent that inhibition, TCAS would be instructing you to climb away from every landing to avoid getting too close to taxiing airplanes near your runway.
A similar problem would happen on takeoff.
It appears that the TCAS system did detect the imminent collision. But as noted, it won’t direct a change in altitude if below 1000ft. Directing a plane into the ground is likely not considered the best answer. The collision occurred at about 400 feet, with the helicopter flying into the airliner, or directly into its path. The helicopter was traversing a designated route that crosses the flight paths into the airport.
Indications are that the helicopter was ascending well above the flight level it was allowed, and likely the pilot had also misidentified which airliner it had been asked to pass behind. Plus the helicopter crew were all wearing night vision gear with concomitant lack of peripheral vision.
The air traffic controller had received a collision warning which had prompted him to check that the helicopter had a visual of the plane and also why he gave the direction to pass behind. ATC didn’t seem to realise that the helicopter was higher than the 200ft allowed for it, and there was no vertical separation.
There is a lot to understand. Much went wrong really quickly. But at first brush, the airliner was blameless and probably had no useful options or time. On final approach it isn’t as if there is much wiggle room.
Proximate blame may be easy. But answering the questions about how to prevent a similar accident is going to be hard. Given how chaotic and challenging the airspace the accident occurred in is, it is incredible this hasn’t happened before.
This was my thought that night when I heard it - they wouldn’t see an aircraft directly right and high up (but descending). You can see another aircraft ascending in the often-replayed video and it’s possible the crew identified that as the aircraft to watch for. The video shows them flying directly into the jet, so obviously they did not see it.
I assume even with the TACAS limitations, the position of an aircraft would show on the copter’s screen. It would just take a bit of time and concentration to read the altitude and realize it was close to the same altitude (along with all the distraction of grounded aircraft)? How fast do the blips move on the screen - we were all raised on Hollywood images, of the blips stationary but updating every 1 or 2 seconds, less obvious or instructive than smooth moving dots…
This will be the interesting question - everything they are saying on the news is that the helicopter was supposed to be 200ft altitude in that area. Presumably we will hear something eventually about what the respective altitides actually were and why…
Here is the latest from the Times:
"Clues emerging from the moments before the deadly collision Wednesday night between an Army helicopter and an American Airlines passenger jet suggest that multiple layers of the country’s aviation safety apparatus failed, according to flight recordings, a preliminary internal report from the Federal Aviation Administration, interviews with current and former air traffic controllers and others briefed on the matter.
The helicopter flew outside its approved flight path. The American Airlines pilots most likely did not see the helicopter close by as they made a turn toward the runway. And the air traffic controller, who was juggling two jobs at the same time, was unable to keep the helicopter and the plane separated."
Multiple points of failure are common in these situations. But the helicopter outside its assigned path seems primary to me.