Diversions and revised estimates

What information makes a diversion estimate usable?

The new destination does not erase the aircraft’s current position uncertainty.

Anchor the revision in a reliable position

A diversion begins from the best reliable current position and time, not from the original departure fix. Identify a usable destination and route considering weather, terrain, airspace, runway availability, fuel and clearance. The shortest geometric route may be unsuitable. Aircraft control and immediate hazard management remain priorities while navigation is revised.

Estimate or plot the new true track and distance. Use a wind estimate appropriate to the diversion height, solve heading and GS, then convert heading to the reference used for flight. Compute elapsed time and ETA with an explicit clock reference and date. Select an early feature or instrument fix that will test the new plan.

Solve the new triangle coherently

For a synthetic 60 NM diversion on track 090°, TAS 150 kt and wind from 180° at 30 kt, the wind blows north. Heading must be south of east: 101.54°T gives a 30 kt southward air component. GS is √(150² − 30²) = 146.97 kt, so leg time is 24.49 minutes.

Starting from a reliable fix at 14:20 UTC, initial ETA is about 14:44:30 UTC. A mental estimate of 24–25 minutes is useful, but it should not disguise uncertainty in the starting position, wind or available routing. An accurately solved triangle does not prove the destination is operationally suitable.

Name the first confirming observation

If a distinct crossing feature lies 20 NM along the revised route, its predicted time is about 8.16 minutes after the diversion fix. That gives an early check near 14:28 UTC. If its bearing, location or timing differs materially, reassess both the position and the wind instead of continuing on an untested estimate. A diversion calculation is stronger when it produces a near-term observable prediction, because an error discovered early leaves more options than one discovered at the intended destination.

How does the first check improve the revised plan?

Use observed motion to refine the remaining estimate.

Update time and fuel assumptions consistently

If the first 20 NM of a diversion takes 9 minutes, observed GS is 20/(9/60) = 133.33 kt. At that GS, 40 NM remaining takes 18 minutes. Use the remaining time in the applicable fuel-planning method and reserve assessment; do not retain the earlier, more optimistic GS for one part of the plan while using the observed value for another.

An unexpected delay can come from wind, route extension, position error or timing error. A cross-track offset needs its own investigation and correction. Similar arrival-time arithmetic can result from different physical causes, so distinguish what the observation actually establishes before changing heading or wind.

Give the next revision a test

Choose another reliable fix or feature and predict its time and bearing or range. If it does not agree, avoid building an ever-longer DR estimate while searching only for confirming evidence. Use available radio navigation, ATC assistance and approved lost-position procedures appropriate to the situation.

The interactive diversion triangle does not choose an alternate or assess runway and reserve requirements. It isolates consistency between track, heading, GS and elapsed time. The broader flight decision still needs current information and the aircraft’s approved operating context.

Carry the new fix time forward

Suppose the reliable 20 NM check is passed at 14:29 UTC. With 40 NM remaining and the observed 133.33 kt GS, the revised ETA is 14:47 UTC. Do not add the original planned 24.49 minutes to 14:29; that would count the first portion twice. If a further 10 NM check takes 4 minutes, its 150 kt GS estimate may indicate changing wind or measurement variability. Evaluate the quality and relevance of the evidence before replacing the whole remaining estimate with one short-interval observation.

Can you explain and apply the mechanism?

Four original questions. Work from the stated facts, then compare your reasoning with the explanation.