Equal-time points and ETOPS, explained

If an engine fails on a twin-engine aircraft, it has to land at the nearest suitable airport. So for decades a twin flying for an airline in the United States had to stay within 60 minutes, at its one-engine cruise speed, of an airport it could divert to. That rule kept twins off most ocean crossings and left them to three- and four-engine aircraft.

ETOPS, first Extended-range Twin-engine Operational Performance Standards and now officially just Extended Operations, relaxed it in steps. An operator that meets requirements for the engines' reliability, maintenance, crew training and planning can be approved to fly 120, 180 or more minutes from an alternate. Today most long-haul flying is done by twins under ETOPS approvals of 180 minutes or beyond, and regulators have approved 330 and 370 minutes for particular types.

The worst point on a leg

Which minute count does a route need? The answer is set by the single point on the route that is farthest, in time, from any suitable airport.

Picture flying away from one alternate and towards the next. Time to the nearest alternate rises as you leave the first and falls as you approach the second. Its maximum is exactly where the two times are equal: the equal-time point, or ETP. So the worst point on a leg is always one of its ETPs, and an ETOPS check is a check of the ETPs, not of the farthest point by distance.

Overfly finds them by walking each leg in small steps, noting which alternate is nearest in time at each, and solving for the exact point wherever that changes. It reports only the ones that matter: overland the nearest airport changes every few hundred miles, and those handovers, with diversion times of a few minutes, are correct but are noise.

Two worked examples

New York to London on the great circle passes between Newfoundland, Iceland and Ireland. Overfly's figures, at a one-engine diversion speed of 430 kt in still air, put the worst point between St. John's and Keflavík at 1h 46m from each, with a second ETP between Keflavík and Shannon at 1h 29m. A twin flying that track needs ETOPS 120.

Sydney to Los Angeles is a different scale. The great circle crosses open Pacific, with Pago Pago in American Samoa, Kona on Hawaii's Big Island and San Francisco as the alternates that govern it. The worst points are 2h 36m from their alternates, so the track needs ETOPS 180. Each route page carries this analysis for its own route.

A suitable airport, in Overfly's model, is one OurAirports classes as large with a runway of 8,000 ft or more. An operator's real alternates are chosen with weather forecasts, rescue and fire cover, and opening hours, and its diversion speed is set for its type; treat these figures as the shape of the problem, not a dispatch release.

The wind trap

Here is something worth knowing before trusting a still-air ETP. Put two alternates symmetrically either side of a route and add a wind blowing along it. The ETP moves: towards the alternate you would be diverting into the wind to reach. But the diversion time at the ETP does not change at all.

In one dimension, with alternates at 0 and L, speed V and a wind w along the track, the ETP is where x/(V−w) = (L−x)/(V+w), which gives x = L(V−w)/2V and a time of L/2V. There is no w in the time. The tailwind one way and the headwind the other cancel exactly at the balance point.

So a jet stream can move the place a crew must decide to turn back by hundreds of miles while the diversion clock says nothing has changed. A still-air plan puts the ETP in the wrong place, and its time gives no hint of it. Real alternates are rarely symmetric, which breaks the cancellation and moves the time too. With seasonal winds switched on, the mapper solves the ETPs in the wind; the route pages show still air, so their figures stay fixed and citable.