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Into Thin Air: A problem even in low country

I live in low country. My airport is a measly 15 feet above sea level, not low enough to be subject to flooding in heavy storms, but plenty low. You’d think that means that my airplane sees peak performance on takeoff all the time. Well, maybe on a standard atmosphere day (59 degrees F. / 15 degrees C. and 29.92 Hg / 1012 hPa, with 50% humidity). Unfortunately its been a while since we’ve seen those temperatures.

Most summer mornings I awake at 0600 to an outside air temperature of 77 F / 26 C, barometric pressure around 29.80 and the humidity? At 96% you might as well breathe water as air. What does that do to my aircraft’s performance? Skunks it. Seriously,  density altitude does a number on my high performance machine—and just about everyone else’s, too. (I say “just about” because a few of you out there fly turbocharged or turbo-normalized aircraft, and they cope better with density altitude, up to a point.)

With temperatures and humidity like we have, my airplane performs more like it is at 1,600 MSL, and that is first thing in the morning.  It doesn’t take long for the sun to cook up the air to temperatures approaching 100 F / 38 C, more than doubling the density altitude effect and making it perform as if we were above 3,000 MSL.

The FAA publishes telephone numbers for direct contact with AWOS/ASOS computers nationwide.

The FAA publishes telephone numbers for direct contact with AWOS/ASOS computers nationwide.

This is because hot, wet air behaves precisely like the thin air at altitude: it is tough for the engine to breathe (heck, I find it tough to breathe!). With molecules of air farther apart and separated by H2O, even the flying surfaces are impacted, resulting in longer take off rolls and anemic climb out performance.

How does one cope? First: calculate. Density altitude calculators are built-in to every flight planning software package worth its salt (even NOAA publishes one here). Don’t have one of those handy? Call the AWOS or ATIS at your airport (they have local telephone numbers, you can find them here).  If you are already at the airplane well, just tune in and listen. No weather reporting at your airport? Just about every airplane has an outside temperature gauge somewhere (your oil temperature will be at ambient temperature before engine start). As for the humidity? If you are sweating on the ramp and you aren’t in Yuma, Arizona, you can guess it is well above 50%. Just figure it in at 90% and you’ll be safe.

Now, pull out the performance charts from the Pilots Operating Handbook that you keep in the airplane (you’ve got to have one, it’s the law). Run your finger across the chart per the instructions and it should spit out a required takeoff roll distance. Is your runway long enough? The climb chart will predict your performance—then you have to ask yourself: is that climb rate adequate (remember, you’ll be going up into even thinner air, so that initial climb performance is likely to deteriorate)?

In my corner of the country side most significant obstacles can be out climbed  in the first 300 feet of altitude. Even at 300 fpm climb rate if I’m patient I can get away from terrafirma on a hot afternoon—that is, if I haven’t packed the back of the airplane with passengers and bags to the ceiling then loaded it up with full fuel. These are all variables I can change. I could also opt out of the flight, rescheduling it for a cooler time of day.

And what if I risk it and try the take off?  Look at the NTSB database (www.ntsb.gov). Search density altitude and you’ll find a host of general aviation accidents where high DA is listed as “probable cause.” Many occurred in the summer, often from high elevation airports. Nearly every time the aircraft was overloaded for the conditions and was forced into the air by the pilot. He or she managed to get it flying in the cushion of ground effect, but once the airplane pulled away from that crutch it was stall / spin time. A few pilots managed to resist the overwhelming compulsion to pull harder on the control yoke and did the right thing, which is to PUSH the nose over to a flying airspeed and ride the airplane back into ground effect and onto the ground in a controlled crash. Not pretty, but survivable.

A few years ago I had my first experience in a Redbird simulator. The instructor with me set up the hot/high demo, where I attempted to fly a Cessna 172 off a mountain valley grass strip on a hot afternoon with no wind. Honestly? It was awful. Even with my best short field technique I felt the airplane sinking as I pulled away from ground effect and I instinctively pushed the nose over, pulled the power and jockeyed the airplane to a landing and a ground loop to avoid the trees at the end of the strip. Not pretty at all, but we did manage to keep life and hardware intact.

With practice I learned what that Cessna could and could not do on that little back country strip. It was an education. Want to try it for yourself? Redbird is partnering with the National Association of Flight Instructors and the Experimental Aircraft Association at EAA AirVenture this summer to bring attendees the Pilot Proficiency Center. There you can sign up to brush the rust off or try skills you’ve yet to master in one of 12  Redbird LD simulators (Advanced Aviation Training Devices). The building is air conditioned and the sim instructors are volunteers. And yes, you can log it as  flight sim time with an instructor sign off.

Give it your best. You’ll impress yourself.

Amy Laboda has been writing, editing and publishing print materials for more than 28 years on an international scale. From conception to design to production, Laboda helps businesses and associations communicate through various media with their clients, valued donors, or struggling students who aspire to earn scholarships and one day lead. An ATP-rated pilot with multiple flight instructor ratings, Laboda enjoys flying her two experimental aircraft and being active in the airpark community in which she lives.

3 Comments

  1. I regularly operate out of high DA airports–just the airport elevations are much higher than your highest DAs: 4700′ MSL at GXY, 5000′ MSL at FNL, 7300′ MSL at LAR, 6100′ at CYS, etc. The single most important item is to properly lean for the DA (excepting turbo’d engines). Too many pilots who trained and fly out of lowland airports haven’t the slightest idea about leaning for take off, but it makes a huge difference in take off performance. The second most important item is to use less of a pitch-up for rotation, so that the airspeed will not deteriorate as the airplane leaves ground effect.

    So far, the highest DA at which I’ve taken my airplane off was 12,100′ from Leadville, CO, elev. 9935′, a 6400′ long runway. Mine is a 1963 Cessna P172D with a 180hp Lycoming/CS prop. The mixture control was almost out to idle cut-off. On that take off, the airplane was lightly loaded–just me and my dog, 3/4 tanks, survival kit, not much else. We rolled roughly 2/3 of the runway before lifting off at about 75 mph IAS, then kept it in low ground effect until 90+mph IAS, before climbing out at something like 150 fpm. The pitch up was barely above straight and level to maintain 90 mph.

    There is no reason to fear high DA situations, but like most circumstances in aviation, they must be respected, by using proper technique, limiting weight, using proper airspeeds, etc.

  2. “Well, maybe on a standard atmosphere day (59 degrees F. / 15 degrees C. and 29.92 Hg / 1012 hPa, with 50% humidity).”

    Everything I’ve read and been taught (including the international definition) suggests that a “standard day” means 0% humidity.

  3. I’m with Scott; pretty sure the International Standard Atmosphere (ISA) is 15 deg C & 0% humidity. Yet, we all know that humidity impacts air density–which leads us into the murky science of applying ‘fudge factors’ to compensate for it. Here’s what AOPA says about humidity & density altitude (shortened a bit):

    “Performance charts for small aircraft use only air pressure and temperature to calculate density altitude. But the amount of water vapor in the air-its humidity-also affects its density. Contrary to what you might think, adding humidity makes the air less dense. Humid air is lighter than dry air. Here’s why:

    Equal volumes of any gas at equal pressure and temperature will always have the same number of molecules, no matter what the gas. In other words, a cubic foot of zero-humidity air at 59°F and 29.92 inHg pressure will have the same number of molecules as a cubic foot of air with 100-percent relative humidity as long as the temperature and pressure remain 59°F and 29.92 inHg.

    Because the number of molecules per cubic volume stays the same as water vapor is added to perfectly dry air, the water molecules have to displace other molecules making up the air. Air is mostly nitrogen and oxygen. Molecules of both of these are heavier than water molecules. As the air grows more humid, relatively light water-vapor molecules are replacing heavier nitrogen or oxygen molecules, making the air less dense.

    Density differences caused by humidity are relatively small. The small decrease in density caused by humidity is why pilots of small, piston-engine aircraft can ignore it in performance calculations.

    Humidity has a bigger effect on engines than on lift, because engines need oxygen to combine with the fuel that’s burned to supply power. Any water vapor that replaces oxygen in the air will reduce an engine’s power output. Because of this, if the humidity is high, it’s probably a good idea to increase the “fudge factor” when doing takeoff distance calculations.”
    Source: AOPA Flight Training Magazine (Sept 96), The Weather Never Sleeps: Density Altitude.

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