September 2008 Volume 51 / Number 9
Safety Pilot: Safety envelope
Bruce Landsberg has been executive director of the AOPA Air Safety Foundation for 16 years.
You’ve heard about flying to the “edge of the envelope,” a concept that is typically introduced in primary training to explain aircraft performance and capability. Fly too fast and you’re out of the envelope, ditto too slow. Critical angle of attack, weight or balance, and almost any performance measurement can be graphed or charted since flying is only applied physics. The envelope concept is simple and seductive.
Going outside the flight envelope is stupid, dangerous, and often invigorating—the forbidden fruit. Test pilots are paid to find the edges and that’s how envelopes are drawn. During the dawn of jet and space flight in the early 1950s and ’60s, envelope exploration was the happening thing at Edwards Air Force Base. The base street names are somber reminders of those who went beyond then-undefined boundaries and didn’t make it back. Even today, it is a high-risk occupation to find the limits for an aircraft to become certificated.
But is it safe to be just inside the flight envelope poking at the edges? That depends. Are you up to managing the machine where its flight characteristics may be markedly different from what you’re used to? Is the aircraft itself in the same airworthy condition as when new? Are the environmental conditions the same as the test conditions or do they just look like it?
It’s prudent to add another line inside the aircraft limits considering these factors and based on your limits, either skill or personal risk tolerance, as a “safety envelope.” Call it personal minimums if you prefer, but I like graphical representations. Unlike the hard line drawn by engineers for various aircraft parameters, your safety envelope should be flexible. It will be closer to the aircraft limits in some areas and farther away in others. It needs to be regularly redrawn as your skill, proficiency, and knowledge ebb and flow.
How far do we need to go into the flight envelope? That depends on how the aircraft is used. Military flying, bush flying, and competition aerobatics are areas where pilots must wring every bit of performance out of the aircraft and themselves. But for routine GA flight the odds are you don’t need to be pushing the odds. How often do you drive your car up to its limits of speed, cornering, or braking? Probably not often, if ever—usually there’s no reason.
The safety envelope concept brings up another cliché about using your superior judgment so your superior skill isn’t needed. As I’ve said before, with enough bad judgment any level of skill can be overridden. Here is where the airlines and the corporate flight operations shine. They take as much judgment and temptation out of the cockpit as possible. By setting conservative rules that keep flights well away from the edges, all the money they spend on top-notch pilots and training is just a lagniappe (extra). Their safety record supports that. The tradeoff comes by giving up some flexibility and with a significant training investment. Long-employed pilots often consider how they would explain an action to the chief pilot, assuming they survive. Sort of dampens the spur-of-the-moment impulses.
The airlines learned early on that creating edge-of-the-envelope practice emergencies was expensive. When jets were first introduced, far more were lost in training for engine failures on takeoff and landing than in the actual situation. The same thing happened in GA in the late 1960s with multiengine-out scenarios and VMC demonstrations. Simulation, although not cheap, became a far more cost-effective option.
“Normal” emergencies such as a forced landing that could happen at any time or an engine-out on a twin must be reviewed regularly, but how much risk should we put into practicing something that may seldom, if ever, be required such as extreme crosswinds? If you routinely fly in high winds and aligned runways are not readily available, fine. Most of us don’t and the risk is appreciably higher.
So what about that demonstrated crosswind component? The FAA does not consider it limiting, so why should you? It merely represents the meanest crosswind that could be found while the aircraft was in certification testing, but the numbers are generally robust. On the A36 Bonanza that I routinely fly, it’s 17 knots, for a Piper Archer it’s also 17 knots and for the Cessna Turbo 210 it’s 21 knots. Those are significant direct crosswind components. Should you practice up to the limit? Sure, with a competent instructor who will exercise due diligence over the flight and is there to take responsibility if there’s a mishap. Rest assured that if metal is bent on landing however, the FAA will ask why the pilot in command felt justified in envelope exploring if the winds were more than the demonstrated amount. The good news is that there will only be an official inquisition if things go badly. Pull it off and you’re the ace of the base—this time.
All CAR 3 or Part 23 aircraft (certification rules) produced in the past several decades catalog the distance it takes to get up or down above the famous 50-foot obstacle. We all know that this is based on demonstrated data and specified conditions. Note that the testing is done not with real trees but with optical measuring devices, which allows test pilots to fly another day just in case the estimates were a little off. Most of us also know that we won’t duplicate that performance because the test conditions rarely exist in our world: new engine, perfect prop, terrific tires, perfectly level runway, and professional test pilot skill in coaxing the best possible performance out of the machine.
The FAA knows this too when it comes to big aircraft, even though Boeing and Airbus go through extensive testing. So the FAA insists that an additional percentage be added; call it a fudge factor, reality thinking, or whatever gets you to where both pilot and aircraft will consistently perform. This is an extra margin and at the edge of the envelope (think of it as a cliff) the margins are gone.
A personal experience some years ago involved a flying family vacation with the aircraft close to gross weight, which is how vacations are always flown—loaded to the max. The closest airport, only 10 minutes from our destination, was barely within the book performance parameters. The big airport with ample margins was half an hour away. I opted not to put my family at risk. Two years later there was a fatal takeoff accident under similar circumstances to those we would have faced. No mechanical malfunction was identified but something didn’t work out. For me, the risk wasn’t worth the reward.
With landing distance, the same thinking applies. Prepared to bet your life and the aircraft on your ability to get book performance on that day? Better to add some margin, just on the off chance that either you or the brakes aren’t quite up to the task. How much extra distance is needed? That depends on pilot skill and many of us are notoriously bad self-evaluators. Perhaps a little objective evaluation is in order—that’s what recurrent training and a good flight review is all about.
None of this is intended to justify sub-standard pilot performance. In too many cases accident pilots don’t show even basic levels of proficiency or judgment. Draw your safety envelope with care and work to reasonably expand it, but be sure the risk is worth the reward. At the margins everything has to work perfectly, and anyone who’s spent much time around either humans or aircraft knows that Murphy’s Law (if something can go wrong, it will) is for optimists.