Critical point is a point at which it takes equal time for an a/c to reach either destination or departure point.
It is a point lying on a route at which it takes equal time for an a/c to either proceed to the destination or to return at the departure airfield. It is calculated for the sole purpose to find out if any mis-happening occurs in flight due to unavoidable reasons, for example passenger sickness, engine failure, in flight emergency etc, what is the best course of action to reach on ground as faster as possible, in between departure and destination airfields.
If the a/c has crossed the critical point, it is quickest to reach to the destination than to return home.
All the passenger flights have the critical point calculated on their flight plans so that it is easier for crew to decide whether the best course of action is to proceed as per plan or to return to the base if situations do not permit the flight to be completed as planned.
DCP = DH ÷ O+H DCP = TCP × G
TCP = DH ÷ O(O+H) TCP = DCP ÷G
Where
D = Distance of route
H = Ground speed of a/c while returning home/ base
O = Ground speed of a/c while proceeding towards destination
G = Ground speed of a/c while returning home/ base with one engine inoperative.
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Point of No Return
Point of no return is calculated solely for the purpose of calculation of the fuel on board, it is strictly the distance an a/c can fly to and come back with reference to the fuel on board, it is in simple words a calculation of the distance up to which an a/c can go to and comeback with a stipulated amount of reserve fuel.
All the passenger flights have the point of no return calculated on their flight plans; it is calculated so that the crew operating on a deserted area with only one airfield available can decide on reaching a specified point whether it is safe to proceed to the destination. For example if in the event of the destination airfield gets closed due to unavoidable reasons, and the a/c has not yet reached the point of no return the a/c can safely return home with the stipulated amount of reserve on board.
TPNR = EH ÷ O+H TPNR = DPNR ÷ O
DPNR = EOH ÷O+H DPNR = TPNR × O
ENDURANCE = FOB-RES ÷ F/C
Where
E = Endurance of a/c
H = Ground speed of a/c while returning home/ base
O = Ground speed of a/c while proceeding towards destination
F/C = Fuel Consumption
RES = Reserve of a/c (Stipulated amount of fuel termed as reserve)
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