AC Schedules....

Thu Jul 20, 2000 1:24 am

Looking at the schedules on-line for Air Canada from YVR to YYZ, I noticed that the travel time listed for each of the flights is different (e.g. looking at all the flights on one particular day) - even for flights operated using the same equipment. The difference is really small (+/- 5 minutes: AC3993 vs AC3997), but can someone tell me how they come up with the flight length #s reported on the schedule? Are they averages from the previous scheduling period? or built in estimates of ATC delays based on time of day?

Posts: 139
Joined: Wed Dec 22, 1999 10:28 pm

RE: AC Schedules....

Thu Jul 20, 2000 5:49 am

One reason may be, that they cannot have two similar times as landing times in their computer system. Another possibility is, that they only got slots for the times they published, and that is what they officially stick to. It can also be, that they add some 5 minutes reserve block for flights that departed in one of the banks of arrived in one of them.

Just by looking at the timetable it is not realistically possible to know the exact answer to your question, but one of the reasons above, or a combination of these is very probable.
Posts: 1008
Joined: Fri Nov 05, 1999 11:28 am

RE: AC Schedules....

Thu Jul 20, 2000 10:47 am

Richie has touched on the right answer.

The reason that you have different "block" times is usually due to slot allocations at the origin and destination ports. Times can also vary from season to season, depending on the strength of the winds.

Block times (time from gate to gate) and flight times (time from liftoff to touchdown) are determined on a) mathematical calculations and b) experience.

For instance, an airline that starts a new route would use a standard furmula to calculate the block time based on equipment, winds, etc. When the flights have been operated for a season or a year the ontime performance can be evaluated. If the actual time is exceeding the scheduled block time then you firstly see if there is a problem causing the longer time, and whether it can be fixed. If it is out of the airlines control then the only thing to do is add time on to the block time (normally in 5min increments).

Hope this helps,
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Joined: Fri Jan 21, 2000 11:51 am

RE: AC Schedules....

Thu Jul 20, 2000 1:38 pm

AA bases block times (gate-to-gate) primarily on historical data.... a lot of historical data. Every AA cockpit crew is supposed to enter all delays encountered into SABRE database via ACARS. The assumption being your flight is the only flight in the world.

On a brand new route there is no historical data so published schedules are based upon aircraft performance data and guesswork about expected delays. After a month of daily flights, the historical data will start to take priority.

For example: I flew AA's first flight DFW-BDL when it changed from B727 service to MD80 service. We were _required_ to fly flight plan speeds, altitudes and routing as much as ATC would permit. Actual block time was 20 minutes less than scheduled (scheduled based upon B727 historical data). By the end of the first month actual block times were within 1 minute of forecast block times. After that first month we were free to adjust speeds, altitudes and routing as pilot desired.  
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