May the G-force Not be With You

By Rod Machado

This is a actual letter I received from a student pilot along with my reply.

Dear Mr. Machado:

I don't know what to do. I'm a student pilot whose instructor insists that the turbulence we feel during flight won't hurt the airplane. Is this so? Turbulence scares me. I'm afraid it will damage the airplane. It also makes me sick. I don't like it. I'm also afraid I'll throw up on my flight instructor in turbulent conditions. What do you think?

Signed: Ready to Earp!

Dear Mr. R. T. Earp:

I don't think you should earp on your instructor, even if he deserved it. Carry an earp-sack with you. If you don't have one handy, but are flying a Piper aircraft, open the small rectangular window on the pilot's side. Place your mug up against it and heave-ho. The only tell-tale clue will be a rectangular ring around your face that will betray the large smile you wear upon entering the flight school. If you're flying a Cessna, don't even think about earping out the big side window, unless you're wearing a HAZMAT (Hazardous Materials) suit. You only have to try this once to realize it's a really bad idea. Now on to the real problem of how turbulence affects the airplane.

Believe me when I say that no one, and I do mean no one likes turbulence. If they say they do then that's probably the testosterone talking. Yes, some folks get used to it but that doesn't mean they like it.

Your instructor is absolutely correct. Turbulence isn't likely to hurt the airplane. But that doesn't keep pilots from imagining that it will. The fact is that airplanes are very strong. Here's a mental experiment that may give your comfort.

Imagine loading your average Cessna 172 in the positive-g condition by turning it upside down. Have 57 men, each weighing 170 pounds, stand evenly spaced across the wings. The wings should flex to their elastic limit without damage. When the men depart, the wings should return to their prestressed position (don't try this at home, even if it is a rental).

This is a demonstration of loading an airplane to its limit load factor (+3.8G's for a normal category airplane). When stressed to but not beyond this limit, the aircraft structure should experience no damage whatsoever. How likely is the average airplane to reach or exceed this g-force limit? On a rare occasion, it happens. When and where it happens is no real surprise.

A NASA study on gusts and maneuvering loads indicated that exceedance of an airplane's limit load factor caused by pilot-applied control inputs, was primarily confined to aerobatic flight, aerial applications and instructional operations.

It's no surprise that an aerobatic pilot, confined to a designated cube of airspace, might yank and bank to keep the airplane within that cube. Aerial applicators are similarly self-restricted by the size of the field they're dusting. Both operations can lead to high load factors as pilots maneuver to remain within these self-imposed zones.

Students are no less likely to be rough on the flight controls, which is the primary reason instructional flights experience excessive load factors. Once, during stall practice, I told my student to lower the nose. He lowered it by shoving the control stick full forward. I grabbed a pencil off the roof and made a note to use more adjectives the next time I discuss stall recovery. Fortunately, we were way below the airplane's maneuvering speed, resulting in a relatively small negative load imposed on the airplane.

This same study indicated that commercial survey operations (e.g., forest and pipeline patrol airplanes) were most likely to experience turbulence or gust-induced exceedance of the design flight envelope. This isn't surprising because these folks constantly fly from 50 to 1,500 feet above the surface where mechanical turbulence is greatest.

Here's how I'd interpret this. First, the loads imposed by aerobatic flight, aerial applications and instructional operations are controlled by their respective pilots. So, be gentle with those controls. Mother Nature, however, is primarily responsible for the loads imposed on commercial survey operations. Therefore, it's best to avoid flying close to the surface when excessive mechanical turbulence is present.

The fact is that airplanes can exceed their design limit loads, but it's almost always the result of something the pilot did or didn't do. Mother Nature is seldom directly responsible for the act.

It's usually a loss of airspeed control that leads to in-flight structural failure and this normally occurs in or near IMC (instrument meteorological conditions). Vertigo, autopilot or instrument malfunctions are a common cause of runaway airspeeds. Operations beyond the airplane's red line might easily lead to divergent flutter and other assorted ills. The bad news is that all bets are off when operating at these speeds.

The good news is that in-flight structural failures in VMC (visual meteorological conditions) are extremely rare. Clear air turbulence isn't likely to damage your airplane as long as it's flown at or below maneuvering speed. (In turbulence, my preference is to fly 10 to 15 knots below the airplane's maneuvering speed. Remember, maneuvering speed decreases with a decrease in weight.)

So the real enemy isn't turbulence; it's your psychological reaction to turbulence. I recall one occasion with a student when, after a nasty series of gusts, he looked out the left window, followed immediately by a peek out the right window, followed by a nod. He was checking to make sure the wings were OK (as if he wouldn't already know). Of course, I was checking them too. We eventually agreed that he'd keep an eye on the left wing and I'd assume responsibility for the right one and we'd immediately report to each other if one was missing.

That's the way our minds work when rattled by turbulence. The problem pilots have, especially new pilots, is that they don't have a way of calibrating how bad the turbulence is. That's why a g-meter is one of the best investments a pilot can make for calibrating his G-Butt sense.

During informal surveys taken in my seminars over the the last 40+ years, pilots rarely report g-forces induced by turbulence to be greater than +2.5Gs or less than +.5Gs (remember, in straight and level, unaccelerated flight, you experience +1.0G). This is well within the airplane's limit load range. To be frank about it, you'll probably pull more G's on landing that you'll ever experience in the air.

Another bogeyman pilots fear is being turned upside-down in an airplane by turbulence. Once again, our imagination works against us. A famous Indy race driver once suggested that the secret to successful racing is a lack of imagination. Imagine that. Skilled racecar drivers can't afford to mull on mashed metal. The existential anxiety provoked by their muse might easily immobilize them.

Pilots experience similar anxiety at thoughts of being flipped over by turbulence. Yet this is not likely to happen. In other surveys, I ask this question: How many of you have been flipped over in an airplane by Mother Nature induced turbulence or gusts? Occasionally, one or two people will raise their hands. I look at them and say, "You were flying in a mountainous area with winds 30 knots or greater, right?" Almost all say, "That's right!"

So there you have it. It's very unlikely that turbulence will flip your airplane over. If it does, then flip it upright. If you don't know how, then find a qualified instructor to show you how (in an aerobatic airplane, of course). It's also unlikely that these same atmospheric phenomena will damage your airplane as long as you avoid excessive airspeeds. It's still a good idea to purchase that g-meter and recalibrate your derriere. In the process, you'll better understand turbulence, which may help you save that same derriere someday. Above all, never let fear take the helm. Never let it stop you from flying your airplane. Never!

Mr. R. T. Earp, I know how you feel. Turbulence can take the fun out of flying just like car alarms took the fun out of parallel parking. No one really likes it, but we can learn to live with it by keeping our imagination in check.

May the G-force not be with you.