A TEMPO Group That Covers the Whole Diversion Window

Compliance time (the rule): the diversion-time threshold is the still-air time at the approved one-engine-inoperative cruise speed:

1 100 nm ÷ 420 kt = 2.62 h = 157 minutes - inside 180 by 23 minutes.

Fuel time (the planning figure):

420 − 45 = 375 kt ground speed; 1 100 ÷ 375 = 2.93 h = 176 minutes.

Both numbers are legitimate but they answer different questions. Wind is used to compute fuel; still air (and ISA) is used to compute the threshold. Quoting 176 minutes in a discussion about the 180-minute approval is a category error - it makes a compliant flight look marginal, and it invites a payload penalty or an unnecessary re-route. Conversely, planning the fuel off the 157-minute still-air figure would understate the requirement by about 13 %, which is the direction that actually kills people.

One further check: the threshold applies at every point in the ETOPS segment, not just the equal-time point, and both ends of the segment must lie inside 180 minutes of a suitable alternate.

No - and the reason is structural, not a matter of interpretation.

The TEMPO runs 6 h 30 to 9 h 30 and covers the ETA of 7 h 30. Inside it, the forecast is 400 ft broken / 1 200 m, both below the 520 ft / 1 600 m LNAV minima.

A destination alternate is judged at a known ETA. An en-route alternate is not: the aircraft may need it at any time from the point where it is committed until the next suitable airport is within reach. Here that is roughly from the equal-time point (about T+6 h) to the exit from ETOPS airspace. You must be able to use KMRW at the worst moment in that window, not at the planned ETA - so a TEMPO that covers the window disqualifies it just as surely as a permanent BECMG would.

Options, in order of preference:

1. Re-route or re-time so that another aerodrome lies within 157 minutes still-air of every point on the ETOPS segment. With no airport closer than 1 100 nm, adding a further 240 nm breaks the threshold outright - so this usually means a different track, not a different alternate.
2. Delay departure so the aircraft is at KMRW after 9 h 30. Check that the whole window moves outside the TEMPO, not just the ETA.
3. Carry holding fuel. Do the arithmetic before proposing it: 2 h of holding at about 2 300 kg/h is roughly 4 600 kg, against a 500 kg margin. That is several tonnes of payload or a cancelled sector, not a trim adjustment.
4. Cancel or replan the city pair.

Whichever is chosen, the conclusion is the same: the flight cannot be released as planned.

What 500 kg means: it is about 3 % of a figure that is itself a worst-case computation - an engine failure, a depressurised descent and cruise with anti-ice, to a specific alternate. That is not a margin; it is rounding. It evaporates on any of the following:

• a heavier landing weight than planned (late cargo, more passengers);
• a lower one-engine-inoperative cruise level, or icing;
• a stronger headwind than forecast, or a different descent profile;
• holding at the alternate, which is precisely what a TEMPO group tempts you into.

Before signing:

1. Recompute the critical fuel at the actual landing weight and the current forecast wind and temperature - not the planning assumptions.
2. Confirm the operator's ETOPS fuel policy: some require an additional percentage or a fixed time reserve on top of the critical-fuel figure.
3. Check the MEL. With the APU inoperative the depressurised case becomes more penalising and must be recomputed; on many types that also has approval implications for the ETOPS entry itself.
4. Check KMRW's runway condition and RFF category against the one-engine-inoperative landing weight on a 2 400 m runway - a dry-length figure is not the same as the day's figure.
5. Confirm the en-route alternates for the remainder of the track and the reclear/redispatch point, so the plan does not rely on one airport behaving.