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« on: March 21, 2022, 01:06:30 PM »
There have been a few claims in this thread regarding flight tracking/ADS-B and in-flight diversions. Thought I’d collect some thoughts.
First, a short intro.
I’ve been an air traffic controller at London Heathrow for 23 years, In the last 14 years I’ve been working in our Operations department. As part of that I’ve worked on developing and introducing operational ATC ADS-B systems (including comparing accuracy to ATC radar data), analysed and worked with commercial flight trackers (one of my duties is to help respond to noise complaints and queries from local residents using FR24 etc)
A few notes on Flight trackers.
FR24 and other commercial trackers use various methods of determining position.
ADS-B – this is where the aircraft is effectively continually broadcasting a message, and part of that message is its own location.
- Note that in some cases, particularly older aircraft, this ‘own location’ may not be derived from GNSS signals, and may even be determined by what we call DME/DME triangulation (DME – Distance Measuring Equipment – a ground-based radio beacon, often co-located with a VOR navigation aid).
- You only need one antenna/receiver to pick up these signals and plot locations. You can buy these for less than $100 and FR24 and others will send you one for free if you then connect up and feed their network.
- Some flight trackers have a commercial agreement with satellite-based ADS-B providers that feed ATC systems for over the ocean flying, away from ground-based ATC radar cover.
Some aircraft aren’t fitted with ADS-B, or can turn ADS-B broadcasts off. In this case, the aircraft’s transponder (sometimes called SSR – Secondary Surveillance Radar) will be used. This is the aircraft sending out signals in response to a pulse from a ground-based ATC radar. This response from the transponder does not include the aircraft’s position, but ATC radar will merge the information received with the Primary radar (energy being reflected back from the aircraft received at the same time as the transponder response) to provide location on ATC radar screens. Flight tracking apps (with one exception – I’ll come to later) don’t have access to this, so they use a multilateration system. Mode S transponder receivers again are pretty cheap to buy (or provided free as above), and FR24 et al use triangulation to provide position data on non-ADS-B aircraft.
The exception that I know if is an iPad-only app called ‘NATS Airspace Explorer’ which is a flight tracking app provided by NATS, the main ATC provider in the UK. It takes a feed from the actual ATC radar system (anywhere in UK airspace above about 1000ft apart from over mountains) rather than ADS-B. It’s what controllers are seeing on their screen in real time.
There are some aircraft that won’t show up on FR24 et al even if they are broadcasting ADS-B. Aircraft owners can apply to be on a list of suppressed aircraft, this is mainly done for commercial or security/govt. sensitivity reasons.
However, the website-based tracker ADSBExchange.com does not use any filtering. It’s a crowdsourced organization and those suppressed aircraft will show up if they are using ADS-B.
Accuracy of ADS-B.
As I said earlier, ADS-B information is now being used operationally in many countries, especially in the USA. I think now around 150 US airports use ADS-B info overlaid on their radar screens, and, as the ONLY source of position information over the world’s oceans out of ATC radar cover. Bearing in mind that you or I could buy an ADS-B receiver to feed flight trackers I’m not sure on what basis any government organisation could ‘interfere’ with that data
In-flight diversions
There was a comment earlier on the thread:
“Aeronautics credentials are not required to connect three points on a map.”
In-flight diversions are a lot more complex than just diverting to the ‘nearest airport’, as some (including the author of that ‘book’) seem to think.
Most international airlines use a service based in Phoenix, AZ called MedLink MedAire. This is a 24/7 operation staffed by ER doctors which is the first port of call for aircraft in the air when there’s a sick passenger.
They will advise the airline on the best course of action.
Factors that need to be considered in choosing a diversion airport:
• If it’s a medical diversion, is the patient stable? Is it every second counts?
• Medical care facilities at the airport, and wider area
• Runway length, runway strength, taxiway strength, anticipated aircraft landing weight
• Weather now and forecast
• Air traffic control provision
• Airport fire fighting and rescue cover
• Fuel and aircraft servicing provision (hydraulic fuel, lubricants, steps to reach the aircraft, baggage facilities, towing capability)
• Does the airline have contracts in place with companies at the proposed diversion airport?
• Engineer/mechanic provision
• Flight crew duty hours
• Relief crew position
• Passenger services (immigration/customs)
• Accommodation for crew and passengers
• Distance from aircraft (to be comfortable for passengers, most airliners require about 10nm for every 3000ft of altitude. The descent rate could be increased somewhat if it was an emergency, but you’re still talking nearly 100nm from 35,000ft).
• Passengers on board (do they need visas to land at proposed diversion? Will they be kept on board for hours while the situation is sorted out?).
• Are there vulnerable passengers on board (i.e. people whom one country would be very keen to get their hands on? (for example the relatively recent incident over Belarus)) .
• Political concerns (i.e. A US flight might go another 20-30nm to avoid diverting to Iran and making it to UAE).
These are factors I have just come up with, and no doubt there are many more to be considered.
To second guess why a crew made a decision to divert to a particular airport with no knowledge of most of these factors (as the 'book' does) is naive in the extreme.
Happy to answer any questions on the above, if I can!
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