Aviation has traditionally cut itself a fair amount of slack to ensure safety is maintained when the unexpected happens. But having any slack in the system is totally at odds with today's business thinking, so the old philosophy is being challenged.

That's fine, as long as those tasked with cutting back the slack fully understand why it was designed into the system in the first place, and carefully think through all the downsides of lean operations practices. Nowadays the unexpected happens less often than it did, but it still happens. The definition of the word "unexpected" also matters. For example, engine failures are rare but, statistically, they cannot be defined as unexpected.

There are certain types of trimming that are easy and safe to do. If part of the margin that used to be allowed for, say, calculating contingency fuel was devised just because the systems for estimating the fuel required were not accurate, that part of the contingency can be eliminated completely when fuel requirements can be calculated more accurately. Lots of other margins can also be rolled back because of increasing accuracy: margins set purely to allow for navigational inaccuracy can safely be narrowed if an aircraft has state-of-the-art navigational systems, if they are backed up.

Meanwhile, SAS, working with the Oxford Aviation Academy at Stockholm, is testing a suite of fuel-saving measures with the aim of setting up a three-day course for pilots whose companies want lean operations.

But behind every decision to cut back is the truth that margins allowed for one contingency can be useful if a different, unplanned problem occurs, although it is probably impossible to work out - historically - how many accidents would have had far worse outcomes but for the slack allowed by the old assumptions.

A classic example of the latter is human error while calculating take-off performance. Electronic "flight bags" can make this more accurate, but they are as vulnerable to incorrect entries as the old system, if not more so. This vulnerability is compounded by the lean, modern practice of taking off with derated power, because calculations are effectively based on using the whole runway length in the event of an abandoned take-off at V1. If margins are to be eliminated, mistakes must be, too.

Although the era when aircraft commanders could, without consulting, decide to load extra fuel "for the wife and kids" has ended, margins are still needed. The industry has to decide what is acceptable, otherwise lawyers will decide that after the event.

Source: Flight International