Eddie Sez:

As pilots we are expected to have a certain level of ego to survive. Without an ego, what sane man or woman would be willing to trust his or her fate to the mechanical and electrical wizardry inside any modern aircraft? Even today’s most basic trainer aircraft would defy all logic and common sense to most people before 1903. But left unchecked, this same ego can be a pilot’s undoing. Keeping within known limitations requires a level of discipline. Exceeding those limitations is a form of arrogance that must be guarded against. The problem is that many of our peers have the wrong idea behind where these limitations come from.

What follows are quotes from the source documents, listed below, as well as my comments in blue.


“Engineers pad all limits by 50%”

Limitations can be padded but rarely by such a large amount. Consider your maximum speed limitation for a ridiculous example. With a 50% pad, a 0.80 Mach limit would then become 1.20 Mach.

When an engineer does “pad” a limit, it has more to do with mathematical certainty than adding safety factors. Let’s say you know a part will fail at 50 lbs. of force but the gauge used to determine this was graduated in units of 2. So the part failed under test conditions when the gauge read 50 but that could have been as low as 48 or as high as 52. The engineer writes the limit as 50 lbs. +/- 2 lbs. The aviation manual writer interprets this as 48 lbs. So yes, you have a pad, but it is only 4% in this case.

You don’t know how much of a “pad,” if any, was used. You should therefore treat the limit as just that, the limit.


“We’ve exceeded this limit before”

The complexities of even the simplest aircraft defy our abilities to consider all the interrelated inputs needed to determine a single limit. It could very well be that on a given day a takeoff weight limitation was easily exceeded without ill effect. But on the very next day in a gusty crosswind the same weight could cause an unforeseen change to minimum control speeds. You just don’t know.

Not all limits are linear. For example, let’s say your stall speeds decrease 10% when increasing flap settings from 10 degrees to 20 degrees. Does that mean they decrease another 10% when going from 20 degrees to 30 degrees? No, the change in camber to the wing may not be affected by the same percentage. And even if it was, that doesn’t mean the airplane can achieve the same decrease in flying speed.

Not all limits can be extrapolated. There is no requirement for manufacturers to test every possible scenario. For example, if the manufacturer certifies an aircraft to takeoff with only two possible flap settings, there is no requirement to test a takeoff with other flap settings. You cannot assume a zero flap takeoff is possible based on the performance of the airplane with greater flap settings.


“The limits were determined by regulations that didn’t consider this airplane”

It is true that many limitations are established by regulatory fiat and the manufacturer must simply achieve the limit required under certification. But under these conditions, you have no idea what the real limit is.

Crosswind limits on a transport category aircraft, for example, are stipulated by 14 CFR 25:

[14 CFR 25, ¶25.237] A 90-degree cross component of wind velocity, demonstrated to be safe for takeoff and landing, must be established for dry runways and must be at least 20 knots or 0.2 VSR0, whichever is greater, except that it need not exceed 25 knots.

So let’s say your manual has a demonstrated crosswind limit of 27 knots. What kind of pad do you have? You might have a very large one. The aircraft might be capable of 40 knots but the manufacturer decided 27 was what the launch customer wanted, that was good enough under Part 25, and that’s all they would sign up to. Or it could be that the aircraft’s actual limit is 27 and just one more knot of wind will be unflyable. You don’t know.


“The test pilot said the real limit is ____.”

The operative word in the title “test pilot” is the second one, not the first. A test pilot is just a pilot and no matter how many schools he or she attended and no matter how many test flight hours he or she has, he or she is not infallible. Flight tests are conducted under the constraints of time and money and cannot be expected to discover every little quirk of the airplane.

Just because a test pilot, on a given day, was able to extract more performance than a published limit doesn’t mean the same can be done on another day. The fact the person making the claim has that “test pilot” title doesn’t make his or her claim any less unwise.


The Bottom Line

Perhaps no area of learning from experience can be more harmful than the wrong lessons learned when deviating from limitations without ill effect. Yes, your limitations may have a pad to them. Yes, you may have evidence that under certain conditions a limit has been exceeded without any noticeable impact. But you are operating with a higher degree of uncertainty than the designers intended. The advantage to adhering to published limitations is you know many thousands of flight hours have been own using the same limits. Once you exceed these limits, you are on your own.


Book Notes

Portions of this page can be found in the book Flight Lessons 3: Experience, Chapter 3.


References

14 CFR 25, Title 14: Aeronautics and Space, Airworthiness Standards: Transport Category Airplanes, Federal Aviation Administration, Department of Transportation