|Quoting skymiler (Reply 3):|
Exactly how is dispatch reliability calculated? Leaving gate within 15 min of schedule if an mx item is encountered?
Would be very interested to know ...
One of the big problems is that there are many different ways to calculate it and each airline and manufacturer does it differently.
When I worked on aircraft reliability numbers, dispatch reliability was calculated by factoring in all flights that had a delay of any length on departure for a maintenance reason. That led to numbers of about 95% dispatch reliability across a fleet of aircraft.
The problem is, what delays are factored in and what aren't. At the airline I worked at, if a passenger spilled a glass of water and soiled a seat cushion causing departure (release of parking brake) was late at all, it would get coded as a maintenance delay since maintenance is responsible for replacing the seat cushion. This would affect dispatch reliability. Similarly a flight attendant breaking a stowbin latch due to a bin being overstuffed would affect dispatch reliability. That might not make a lot of sense, but at the airline it is up to engineering to find a better way to make replacing the seat cushion faster or deferring the bin latch so that it would not impact the operation and thus was a maintenance issue. Other delays such as off loading a no show passenger's luggage would get coded to airport operations as it was up to them to figure a way to find the bag faster or notify the ramp to begin searching earlier. At that airline, the coding was based on what group would be responsible for fixing the problem, so the overall dispatch reliability numbers were broken down into sub categories.
Another factor is that multiple dispatch rates are examined. An airline might look at Delay > 0, Delay > 15 min, Delay > 120 min, cancellations, Arrival > 15 etc. Each number has value and is reviewed by the airline's reliability department. There might be quite a few delays of a few minutes caused by situations similar to what I mentioned above. While high in count, these don't actually cost the airline very much and can be fixed quickly. Conversely the delays over 120 minutes are much more carefully scrutinized as these can be caused by different mechanical problems. A delay caused by Flight Controls for example is almost never going to get fixed in a few minutes. Those types of delays usually result in missed connections, overnight delays, etc which are quite costly. Therefore the airline has to break apart delays and might tolerate 1 Flight Control delay for every 5 Cabin Furnishings delay.
Boeing takes airline data and filters it to create a dispatch reliability rate for the fleet. They also allow each airline to compare themselves to other operators. The discrepancies are huge, and show that calculating dispatch rate is not a perfect science. Boeing tries to filter out the airline specific elements to calculate the percentage of flights that the airplane itself is available at departure time.
So in the end, every airline does it differently and it is very hard to compare evenly. Some 777 operators have a 99.8% dispatch reliability that they quote while others might be at 97%. The airline at 97% isn't having its airplanes fall out of the sky, and statistically an enhanced reliability program and more proactive maintenance cannot account for the differences. That is with the same model. If you try to compare A vs B on numbers quoted from separate airlines, you can throw statistics out the window because it is far to variable to be a realistic comparison.
If you have never designed an airplane part before, let the real designers do the work!