CRITICAL POINT (CP) OR POINT OF EQUAL TIME (PET)

Introduction

Any time a pilot encounters a land ASAP emergency in the air, he should be aware of the PET, so that he could land within the shortest time. This landing could be at the departure, destination or an alternate aerodrome. Flight planning has to cater for the actions that would be required to bring the aircraft down safely on the ground in such a case. PET is the point along an aircraft’s track from which it takes equal time to proceed to two selected bases. This calculation of PET assists the PIC to decide whether it will be quicker to return, if short of the PET; or if beyond the PET, to continue onwards. On extended range twin operations (ETOPS), PET can play a vital role in planning of the flight. Many other factors, like facilities at the intended base, spare part availability,   replacement crew, etc., will also have to be considered by the Commander before taking the final decision.

The calculation for PET:

Aircraft proceeding from A to B, a distance of D nms. The winds are from B to A, and thus the CP would be into winds or towards B. Distance from A to CP is X and thus distance from CP to B would be (D-X). Ground speed outbound is given by 'O' and the ground speed home by 'H'. Since, as per the definition above, it takes equal time to proceed from CP to A or B, it can be concluded that t1 and t2 are equal, or

t1 = t2, or

x/ h = (d – x)/ o      

(S = ut or t = S/u)

                                                                                xo = dh – xh

xo + xh = dh

x(o + h) = dh

or x = dh/ o+h

(where x is the distance to CP or DCP)

Time to CP or TCP will be = DCP/ Ground speed out to PET

Engine Failure PET: In most cases, loss of a jet engine would involve “drift down” to a lower pressure altitude where the power from the remaining engines can sustain the aircraft. This would lead to a loss of performance, or lower TAS and ground speeds. The worst case scenario would be a loss of an engine at the PET. In such a case the reduced ground speed out (O) and reduced ground speed home (H) would need to be considered for calculating the PET. However, since we are considering engine out only at the critical point, the time to the PET (TCP) would have to be found by taking the ground speed out with all engines operative.

Effect of wind on PET

• With nil winds or equal winds in both directions (winds at 90° to track), the PET would be midway, as the ground speeds out and home would be same in such cases.
• With winds along track the PET would move into wind from the midway point.
• Larger wind component along track, larger would be the movement of CP.
• Larger TAS, smaller would be the movement of CP.
• And vice versa, in both the above cases.