Into Thin Air

Mountains are not a requirement for high density altitude

July 1, 2000

"On August 22, 1998, at approximately 1245 mountain daylight time, a Cessna 175A was destroyed on impact with terrain after takeoff from Tri-County Airport, Erie, Colorado. The private pilot and two of his passengers were fatally injured; a third passenger was seriously injured. Visual meteorological conditions prevailed for the local personal flight."

Thus begins the terse NTSB account of another unfortunate accident, one that is all too common. The low-time pilot from northern Texas and his passengers paid the ultimate price for a lesson on thin-air flying.

It's important to note that this accident didn't occur at a "mountain airport." That's one of the mistakes we often make when we fly in thin-air/high-density-altitude situations: If there are no craggy peaks surrounding us, we might not recognize that we are in a thin-air environment. The Erie Tri-County Airport is located on the dry plains, 14 nm east of the nearest mountains, the Front Range of Colorado. Yet at 5,130 feet above sea level, the air is thin on most summer days. On the day of this accident, the density altitude was well over 8,000 feet.

NTSB investigators found that the Cessna 175 was slightly over gross, and witnesses testified that the pilot had a tailwind on takeoff. The temperature was 92 degrees Fahrenheit. Worse, the owner-pilot had topped off the tanks just before departure, adding 21.4 gallons of auto fuel.

Don't miss these thin-air cues:

  • Temperature is above standard temperature for the elevation at the departure airport. Once you have determined the standard temperature for your altitude, add 600 feet to your pressure altitude for every 10 degrees above standard to get density altitude. Thus, at 5,000 feet pressure altitude and 91 degrees, the density altitude is approximately 8,000 feet.
  • When you push the throttle to full power, the manifold pressure is significantly less than normal sea-level values.
  • The airplane is performing sluggishly and "doesn't seem right…."
  • You are sweating. If you close the window and begin to sweat, you are probably going to experience high density altitude—even in Florida!
  • None of the locals is flying.

How do we best deal with thin air? The easiest way is by reducing the weight in the airplane. One or two passengers may have to wait for the next ride around the city, or catch the airlines or Amtrak to the destination; extra baggage might need to be shipped ahead. And don't worry about not buying a lot of gas from the nice line person; wise pilots operate only with enough fuel to do the job and plan an additional fuel stop in lower/flatter/cooler terrain after leaving the high-altitude airport.

One thing we must accept when air is thin: Our airplanes become less capable and utility suffers. Just because we are below maximum gross weight and thus "legal," it may not be possible to lift that weight in thin air. Maximum certified gross weight is really only meant for sea level if you expect sea-level performance. A four-place aircraft becomes a three-place or two-place—or even a single-place—as the air gets thinner.

Here's part of the reason: On average, our airplane loses about 3.5 percent of its power for each thousand feet of density altitude. Thus, a new Cessna 172R with a mighty 160-horsepower engine at sea level loses 31.5 percent of its power at a density altitude of 9,000 feet, leaving only 109 horsies to pull you over those trees at the end of the runway. That's about the horsepower equivalent of a Cessna 152. Would you load four well-fed folks into a Cessna 152 and attempt to fly over Mr. Tree off the end of the runway?

If that doesn't get your attention, let's look at that 11,000-foot ridge that you want to cross a few miles away. On a very hot day, the density altitude could easily be higher than 14,000 feet on that ridge, leaving your Skyhawk with a sickly 82 hp to pull you over those menacing rocks. Maybe you'd better make another plan.

Notice that the pilot of the ill-fated Cessna 175 in Colorado managed to make it off the runway. In a lot of thin-air or high density-altitude accidents, the airplane leaves the ground—it's what happens after takeoff that determines the fate of the aircraft and its occupants. The thing we often don't address in all our planning is gradient.

The pilot's operating handbook's performance section gives you enough information to get you only 50 feet in the air—beyond that, you are a test pilot. There is no "book" information in our lighter general aviation airplanes for the segment of climb that begins about when the wheels hit the wells and delivers you safely over all obstacles. The FAA doesn't require it, and manufacturers don't provide it.

Gradient means altitude over distance. An airplane with a 60-kt climbing groundspeed and a climb rate of 500 feet per minute has a gradient of 500 feet per nautical mile. That same airplane at a 90-kt groundspeed will only make 333 feet per nautical mile. Thus, the higher climb speed flattens your gradient and lessens your ability to clear obstacles sooner. If you need altitude to clear obstacles, the best way is to reduce the climb speed to best angle of climb (V X) and fly into the wind.

If we look in the POH of that same new Cessna 172R, it shows that the airplane is capable of climbing at 754 feet per minute at sea level on a standard 15–degree-Celsius day. But, at 5,000 feet pressure altitude, with 35 degrees Celsius, the climb rate sags by nearly half to 455 fpm. That's pretty alarming if you've got a windshield full of trees or rising terrain—and especially troublesome if the airplane flies through hot, sinking air right after liftoff. The gradient for this example is a paltry 364 feet per nautical mile, since the climb speed of the 172R is 75 KIAS. If you have a 2,000-foot ridge to scale two miles from the airport, you are going to have to circle. That might not be comfortable in the low-speed regime in which you will be operating.

Checklist

  1. Look for cues above.
  2. Set the mixture control for thin air.
  3. Make sure you are getting the proper power.
  4. Consider the gradient.
  5. Fly the proper speed for climb.
  6. Map the surrounding area and have lower terrain for an out.

Notice that the Cessna's manual warns, "For operation with tailwinds of up to 10 knots, increase distances 10 percent for each 2 knots." If you put some numbers with that, a fairly light 10-knot tailwind will increase the ground roll and distance over the obstacle by 50 percent! If we apply that to the conditions found at the Tri-County accident site in our example, our new 172 would take at least 4,953 feet to clear a 50-foot obstacle. That's the damage a tailwind does to our gradient: Distance to clear the trees or wires is well beyond the end of the 4,800-foot runway.

The other variable over which we have control in thin-air flying is temperature. No, we can't control the temperature directly, but we can choose to fly early in the day or late in the evening when the air is cooler. You will notice that most high-altitude airports are pretty quiet places right about afternoon siesta time, when temperature peaks. Again, utility of our airplanes might suffer, but the nap during the hottest part of the day will keep you alive longer.

The pilot of the Cessna 175 in Colorado lifted the nosewheel well down the 4,800-foot runway, according to witnesses, and the airplane finally became airborne. With gently rising terrain to the south, the pilot surely had the impression that he was going to fly into the ground with his nose-high attitude and lethargic climb rate, exacerbated by the tailwind. He most likely pulled the nose a little higher—and a little higher, until the wing stalled and the airplane crashed nose-first into a highway berm right off the end of the runway.

Pulling back and raising the nose when the airplane is settling in this condition is a natural, but deadly, instinct. So is pulling back to avoid rising terrain. Instead, it's better to relax back-pressure to allow the airplane to accelerate, or get closer to the ground in hopes of using ground effect. That may be the difference between making it over that rise and landing on it.

Another option in these dire straits is quitting. We don't emphasize that option in our training, and pilots don't tend to consider it when faced with an overloaded, underperforming airplane in thin air. But consider landing the aircraft under control rather than risk stalling—when there are no other options.

There was another big mistake apparently made in this Colorado crash. When investigators examined the mixture control of the 175 it was found in the full-rich setting. You must lean properly—and aggressively—to get maximum performance out of your airplane in thin air. Full-rich mixture is only for full-rich altitudes. Above that, the mixture must be thinned or pulled toward the lean position. On fuel-injected airplanes, we can use the placard showing fuel flow versus altitude near the fuel flow indicator; for carbureted engines, we lean for maximum rpm. And for those airplanes with engine monitors like the JPI and GEM, we can set even more accurate mixtures with the digital information.

So how do we cope with thin air? Study the POH performance section, but look for more than just the takeoff numbers. Consider what happens after takeoff, and think carefully about gradient. Be conservative in your planning. Plan for sluggish climb rates, inefficient propellers, engines with less-than-perfect compression, a wing with a few dents, and lots of bug splats. Then plan to leave a bit of room for ol' Murphy when he throws that 500-fpm sinker at you right off the end of the runway or on the downwind side of the pass.