Estimated Time En Route

Distance divided by ground speed. There is nothing to explain about the arithmetic, and this page exists because of what people put into it rather than what it does with them. The ETE is the number that fuel reserves, watch schedules, tidal gates and arrival slots are all built on, and every error in it propagates into all of them, with the errors and the consequences pointing the same direction.

The speed must be the speed made good OVER THE GROUND. True airspeed is what the aircraft does through the air and it is not this; log speed is what the vessel does through the water and it is not this either. Both are systematically optimistic on exactly the legs where it matters, because a headwind or a foul tide is what makes the leg long in the first place. The wind triangle page turns airspeed into ground speed, and the set and drift page does the same job for a vessel; use one of them first.

The distance has to be the distance you will actually cover, which is a separate trap. A rhumb line is longer than a great circle between the same points, sometimes by four percent or more; an airway routing is longer than either; a vessel beating to windward may sail half again the straight-line distance. And the ETE from this page is time in transit only — climb, descent, taxi, locks, a river bar that only opens near high water, and a mandatory rest period all get added afterward, or they get discovered later.

Finally, a reserve computed as a percentage of this number inherits every error in it, proportionally. If the ground speed was ten percent optimistic, so is the reserve that was supposed to protect against exactly that. Fuel and provisioning reserves are better computed against a deliberately pessimistic ground speed than against a comfortable one with a margin bolted on, and that is not a rule about arithmetic — it is a rule about which way to be wrong.