Hi Capt Lim,
I am an ex-Air Force pilot where I flew transport planes from small turboprops to the Airbus A310. Now I am flying as a corporate pilot, and while surfing on aeronautic matters in the web, I found that term ‘wet footprint'. It seems to be "when flying over water, the maximum distance one could glide, should engine[s] fail, and still end up landing in the water" - so confusing isn't it?.
Could you please give me some more information about it - how to compute it, etc?
Remember my answer on ETOPS (‘Engine Turns or Passengers Swims') here? Well ‘wet footprint' is another term to mean that some commercial aircraft are not allowed to operate along this portion of the route unless it has the ETOPS coverage.
If you have some knowledge on ETOPS (read more here), you are most unlikely to be confused by this explanation ‘when flying over water, the maximum distance one could glide, should engine[s] fail, and still end up landing in the water'. Anyway, the explanation is only partially correct - ‘not maximum distance one could glide' but rather ‘maximum distance flown on one engine'.
Yes, you would end up landing in the water because you were not supposed to fly past the PNR or ‘point of no return' from a suitable diversion airport. PNR is sometimes known as the ETP or ‘equal time point'.
Let's say if you are flying across the water from London to Keflavik in Iceland, you are required to calculate the ETP. This calculation serves as a planning strategy so that if you have an emergency before the ETP you can turn back to your chosen alternate airport at, for instance, Glasgow in Scotland. If the emergency occurs past the ETP, then you must continue on to Keflavik. If you insist on turning back now, you may get ‘wet', meaning, you end up in the water!
Most trans-oceanic flights today would require an ETOPS of at least 120 minutes for any 2-engines commercial aircraft. (Boeing 777 has ETOPS of more than 180 minutes) This means that at any point in the flight, a suitable airport able to handle the particular aircraft must be available on one engine within 2 hours.
How do one calculate the ETP?
The answer is basically as follows:-
ETP = (DxGSret)/(GSret + GSfwd)
D = Total distance for the flight
GSret =Ground speed from the ETP back to the origin (GS return)
GSfwd = Ground speed from the ETP to the destination (GS moving forward at the ETP before you turn around to return)
Today, most computer generated flight plans (CFP) would calculate all these details for you. On the Airbus A320 and above, the Flight Management Computer works that out for you in the air as well.