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IMC Weekend Edition 4 May 2013

04 May 2013 10:40 AM | by IMC News Service (Administrator)
Weight, Balance and Density Altitude

By Peter Conant - This past Wednesday I listened to a small part of the Plane Talk webcast on IMC Radio, with Radek, Derek and Dave discussing performance charts. The topic is near and dear to my heart since I've had a few experiences flying at gross weight (and even over gross) in high density altitude conditions which hammered home the need to "run the numbers" when I am near the limits of the chart. As Derek said at one point, "Flying at gross weight is like you're flying a whole different airplane."

Which is a good way to put it. Those of us who fly out of sea-level fields can get a bit blasé about those performance numbers. On one hot and humid afternoon in Maryland, soon after earning my PPL, I offered a high school friend along with his wife and daughter a sightseeing flight in my Archer. I checked the weight and balance but didn't look too closely at the performance numbers. Sure enough, we were slow to accelerate, took fifty percent more runway to lift off and sort of staggered up in the warm moist air. If you've never had the opportunity to fly at or near gross weight on a hot day (even at a sea level airport) you don't really know how your plane performs (or doesn't perform) at the limits of the envelope. It's truly like flying "a whole different airplane."

Or try this: head out to the Rocky Mountain states and see how things are done. I was in Albuquerque some years ago and was sightseeing in a Skyhawk with an instructor. The runup procedures were unique and something I hadn't seen before. Rather than departing with the mixture at full rich from a high altitude field, the instructor held the brakes, ran the engine up to 2,200 RPM, and then leaned the mixture from rich down to a point where the RPMs increased to around 2,350. He then throttled back, pushed the mixture knob in about 1/8th inch, and said "Now the engine is leaned for max power." I think I had read about this but never seen it done before. And on approach to landing, he never went "full rich" as I expected. Something to remember.

Likewise, flying out of Centennial Airport in Colorado, elevation about 4,500 MSL with a friend and my daughter on a clear cool morning, my daughter asked ìWhy is it taking so long for us to get off the runway?î Same deal. The Bonanza A36 I was flying supposedly had an altitude-compensating engine driven fuel pump so that, according to the POH, the need for leaning at a high altitude takeoff was not necessary. Uh huh. I think I'll do the "leaning on the ground" drill every time I'm in high country.

But the all-time weight and balance winner in my logbook is my ferry flight in a Cherokee Six from Maine to Greece back in 1996. This was a delivery to a Greek buyer who had learned to fly in Massachusetts and bought a plane which he promptly outfitted with new interior, avionics, engine and paint. I've written about this before but thought I would repeat the experience here since it certainly got my attention. My co-pilot and I had an extra 100 gallon ferry tank behind the front seats, so with 180 gallons of fuel we could comfortably fly for fourteen hours, meaning at any point we could do a 180 back to our point of departure. (That's 180 degrees, not 180 gallons.) Anyway, the morning of our departure out of Reykjavik Iceland was chilly and cloudy but with clear skies forecast over the North Atlantic enroute to Norway, passing over the Faroe and Shetland Islands.

We were near or slightly over gross weight with our six hundred pound ferry tank, though we made sure that we were within CG limits and knew we'd be flying out of airports with long runways. We had had no problems flying in and out of Labrador and Greenland the day before so we did not anticipate any problems. The clouds were forecast to contain a "chance" of ice.

Turning east and climbing to our assigned altitude of 9,000 MSL, we entered the clouds at 4,500 feet. And there was the ice. Our plane had the 260 horsepower engine and we were a bit over gross weight, but now we were picking up a bit of rime along the leading edges and wrapping around the VHF antennas strung from the top of the vertical fin to the left wingtip and from the wingtip to the top of the fuselage.

Our rate of climb began to decrease: 600 FPM, 400 FPM, 200 FPM. At 100 FPM, we were barely climbing at 65 knots indicated and 7,000 feet. It felt like trying to coax a reluctant elephant up a ramp! And then came the point where the climb stopped. Any further back pressure only produced a loss of airspeed (60 knots), with a distinct pre-stall rumble felt through the yoke. Time to activate plan B. We were cleared to descend down to 4,000 where we were out of the clouds, ice melting off and clear of terrain. As we crossed the coast and headed out over the ocean, the clouds parted and clear skies prevailed.

Flying over gross weight in IMC while picking up ice over hostile terrain is not one of my fondest memories. Thankfully my "co-pilot" was a pilot in the Norwegian Air Force who had made over a dozen Atlantic crossings. And the only ice we saw during the entire 35 hours of flying was in Iceland. Go figure.


  • 05 May 2013 7:16 AM | Bill Castlen
    One way for us "sea level" folks to get a roughly approximate feel for high density altitude is to try climbing with about 24" manifold pressure. Start at least 1,000 ft AGL with the plane at best climb airspeed then reduce the MP to 24". Since in a standard atmosphere we lose about 1" MP for each thousand feet of climb, 24" would give one approximately the power you could expect at 6,000 ft. (30" at sea level minus 6" equals 24") Clearly, for this demonstration, full power is available at any time, so don't take the demonstration any further that what is safe and comfortable.
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