Met Update, PET and PSR - ATPL 033 — Flight Planning and Monitoring

A navigation plan is perishable: it was priced on a forecast that has since been reissued. The second half of pre-flight preparation takes the latest meteorological information and walks it through the whole plan: the cruising level is re-chosen, the true altitudes are re-checked, every heading, ground speed and time is confirmed or corrected, the fuel is repriced, and two derived points are computed for the route ahead: the point of equal time and the point of safe return.

The most fuel-efficient altitude

The cruising level is a trade: higher usually means a leaner engine, but the wind has a vote. The honest comparator between candidate levels is fuel per ground mile: for each level, take the ground speed the level's wind allows and the fuel flow the manual gives for its temperature and altitude, and divide flow by ground speed. The level with the fewest kilograms per nautical mile wins, and a strong enough headwind aloft can hand the decision to the lower, thirstier level: burning more per hour matters less than spending fewer hours per mile.

True altitude and the clearance recheck

The minimum altitudes of module 1 and 2 were guaranteed on a datum; the day's atmosphere can undermine it. With the latest temperatures the true altitudes are confirmed with the familiar machinery: about 4 per cent of the height per 10°C of ISA deviation, and roughly 30 ft per hectopascal where the QNH differs from the setting flown. Colder than ISA means truly lower than indicated, so the check runs the planned levels against the statutory minimum clearances once more: if the corrected true altitude no longer clears the obstacle datum by the required margin, the level, not the obstacle, moves.

Headings, ground speeds and times

The updated winds re-run the wind triangle of the VFR module for every leg: the wind-correction angle from the crosswind component (crosswind × 60 ÷ TAS, pointed into the wind), true heading from track and WCA, magnetic heading via the variation (variation east, magnetic least), and the ground speed from the head- or tailwind component. The ground speeds then re-price the clock: each leg time is its distance over its ground speed, the total time en route is their sum plus the departure and arrival manoeuvring, and the same arithmetic runs once more for the diversion from destination to alternate, whose time the alternate-fuel component depends on.

The revised fuel plan

New times mean new kilograms. Each leg's revised time multiplies the fuel flow again, the trip fuel moves, and everything derived from it moves too: contingency is re-derived from the new trip, and the alternate fuel follows the recomputed diversion time. The stack of the fuel module is not filled in once; it is kept true against the latest forecast until the doors close.

The point of equal time

Two derived points close the preparation. The point of equal time (PET) is the point on the route from which flying on to the destination and turning back to the departure point take the same time: before it, turning back is quicker; beyond it, pressing on is. With O the ground speed onward from the PET and H the ground speed homeward, the distance from departure is distance to PET = D × H ÷ (O + H), and the ETA at the PET follows from that distance at the outbound ground speed. In still air O and H are equal and the PET sits mid-route; with wind it moves into the wind, because the homebound leg that rides the tailwind can afford to be longer. The speeds must be the ones that would actually be flown: a PET computed for an engine failure uses the one-engine-inoperative ground speeds.

The point of safe return

The point of safe return (PSR) answers a different master: not geometry but endurance. It is the farthest point of possible return to the departure point within the safe endurance, the endurance with the reserves left untouched. The time to the PSR is t = E × H ÷ (O + H) with E the safe endurance, and the distance is that time flown at O. Any wind, head or tail, shrinks the PSR's distance below the still-air figure, because the round trip always pays the wind more on one leg than it collects on the other. The two points are cousins, not twins: the PET asks where help is nearest, the PSR asks how far return remains possible, and depending on endurance the PSR can lie on either side of the PET.