A Complete Manual for Conducting International Flight Operations

Revised Estimate

Revised Estimate

Despite the widespread adoption of CPDLC and ADS-B/C, pilots are often still required to provide time estimates for crossing fixes, FIR boundary’s or

destination airports. After you have provided an estimate for crossing the fix, FIR boundary or destination it is important to monitor the accuracy of the time provided to the ATS agency. If this time varies because of unanticipated headwinds or degraded aircraft performance it is important to advise ATS of a revised estimate.

When Is a Revised Estimate Required?

ICAO Recommendations forRevised Estimates (ICAO Annex 6, Chapter“Change in time estimate: If the time estimate for the next applicable reporting point, flight information region boundary or destination aerodrome, whichever comes first, is found to be in error in excess of 2 minutes from that notified to air traffic services, or such other period of time as is prescribed by the appropriate ATS authority or on the basis of air navigation regional agreements, a revised estimated time shall be notified as soon as possible to the appropriate air traffic services unit.”There seems to be some confusion with respect to when a revised estimate is required. If you ask one pilot they will say 3 minutes deviation if you ask another pilot they will say a deviation of time more than two minutes. The recommendation from ICAO and the conservative answer is the later.


It is important to emphasize that time estimates shall not be attempted to be corrected by increasing speed per mach number technique as explained in NAT Doc 007, as this can result in a loss of separation between aircraft. In the case where an unanticipated headwind or tailwind occurs all aircraft will experience a change in the estimated time for crossing a given fix and for that reason all aircraft will generally need to provide a revised estimate. It is for that reason that ATS will immediately be able to recognize an aircraft that is correcting for the time deviation using speed.


Revision date: July 29, 2015

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