When aircraft are being certified, the manufacturer must test the aircraft in many different ways to define certain levels of performance for many phases of flight, such as take-off distance, maximum rate of climb, landing distance and many more.

There are different types of performance categorisation.

Measured Performance is the performance achieved by the manufacturer under test conditions for certification. It utilises new aeroplanes and the test pilots, and is therefore not representative of the actual performance that any single aircraft might be able to achieve in normal circumstances, flown by a normal pilot.

Gross Performance is the expected fleet average of aeroplanes, given that they are sufficiently maintained and flown as per the techniques detailed in the flight manuals, by a normal pilot (not a test pilot in ideal conditions). This means that any aircraft in the fleet has a 50% chance of exceeding this performance, but also a 50% chance of not meeting this performance..

Net Performance is the gross performance, reduced by the safety margin, to account for various possibilities that could commonly cause reduced performance, such as variations in pilot technique, below-average aircraft performance, etc. The safety margin is set so that the chance of an aircraft not meeting the net performance level is extremely low. The safety margin is not fixed, It depends on the likelihood of the situation occurring, and is set to make sure that the total probability of an incident or accident occurring is at least lower than 1 in 1 million, when a passenger buys their ticket (known as their “remote probability”).

This might mean that the safety margin (between gross and net performance) is large for likely events such as a take-off with all engines operating, but the safety margin is very low or even zero for unlikely events such as an engine failure at V1.