Lesson 73 - External Pressures and Density Altitude

Aviation is full of acronyms. ATOMATOFLAMES, NWKRAFT, IMSAFE, FLAPS, CGUMPS ... It's pretty endless. If you're a pilot and you're reading this, then you know. If you're not, don't sweat it. Just be glad your pilot does when they're in the cockpit. One of the safety acronyms we use is PAVE

Pilot

Airplane

enVironment

External pressure

It's kind of a "go, no-go" checklist that pilots should run through to determine if they should fly the plane. Is the pilot in good shape? Is the plane airworthy? What's the weather? Are there any external pressures which might make you want to fly when it's otherwise not safe to do so? Today's lesson targeted the last two in this checklist.

Today's flight was to be a long cross country flight to Colorado Springs and Pueblo with my instructor in preparation for my eventual solo long cross country flight. Since I have switched from the Grummans to the Pipers, I've had the added benefit of a rear seat in the airplane. Both of my kids have expressed interest in flying with me pretty much since I started this madness over two years ago. While the Cessna 172s I started training in had 4 seats, I was reluctant to fly with my kids because I wasn't comfortable flying. Then I switched to the Grummans, in which I got comfortable enough to carry passengers, except that the Grummans I was flying only had two seats, so with my instructor in the right seat, there was no room for anyone else. I can't carry non-pilot passengers until I get my license. The Grummans got totaled in a hailstorm, so I switched to flying the Piper Archers. Now I have rear seats, willing passengers, and a CFI who says "sure!"

My daughter is fast to play the "eldest child" card whenever possible, so she claimed dibs on the first flight. I wanted it to be something cool, not just flying in the pattern, so I scheduled this long cross country to Pueblo so she could fly with us. She was understandably excited, and I was anxious to make this as memorable of an experience as possible. I figured we'd fly to Colorado Springs, where we'd fly over her grandparents' and her aunt's houses. (They live on the approach to the airport.) Then we'd fly to Pueblo where we'd stop and get some ice cream, then return back to Centennial. It would be a full flight, but a fun one. My desire to fly this for my daughter is a textbook example of an "External pressure" to make the flight.

My instructor had me run a weight-and-balance sheet for the flight since we were carrying an extra passenger. This is a sheet where we total up the weight of everything on the plane, and based on that weight, the outside temperature and atmospheric pressure, and published performance data for the airplane, determine if it's safe to fly, and what kind of performance we can expect. This is the "enVironment" part of the acronym. I ran the numbers, and based on published charts, we were under the maximum allowable weight even with full fuel tanks, and performance would be on par with other flights despite temperatures expected to be a bit on the warm side. Things looked good. On paper.

We arrived at the field, and I got the plane ready while my daughter gleefully snapped countless photos on her phone of the process. My instructor looked over my weight-and-balance sheet and agreed that we were in good shape based on the published data. We loaded everybody and everything up and set out on our journey. I taxied us to the run-up area where things were textbook. I had the flight plan pulled up on my iPad to get us down to the Springs. ATC cleared us to take off on runway 10, which was going to save us a good bit of time in its own right. We decided to try a short field take-off since I'm working on perfecting those techniques. The temperature was around 80 degrees, and the density altitude at the airport was reported at 8,900'. (Density Altitude is the altitude that the plane feels like it's flying at. Warm air is less dense, and higher elevations have less dense air, so the warmer it is, the higher it "feels" to objects flying through them. A density altitude of 8,900' is high, but not unusual for Summer. Plenty of small planes were flying today, and besides, things looked good. On paper.

Lined up on the runway, brakes set, 25 degrees of flaps, full throttle, release brakes, and we're off. The plane seemed a bit lethargic compared to what I was used to, but we're also carrying an extra body and full fuel. We're still below max gross weight, though. We're good. I reach 64 knots and begin to pull the nose back to take off. We take off. Almost immediately, the stall horn sounds. I pitch the nose down to level flight to try to gain more airspeed. Not really gaining much speed. Maybe a little, but I'm also not climbing. I pitch up a bit to try to gain altitude, and the stall horn starts chirping again. Level off, gain a bit of speed, pitch up to climb again, and the stall horn sounds again.

This is only my 5th time flying the Piper Archer, and my third in this specific airplane. I have not had a chance to really get a feel for how these planes perform to know if it's performance--or lack thereof--is operator error or something else. But I was beginning to take this personally that this plane simply did not want to listen to my command to climb. Stall horn. Level off to gain airspeed, climb, stall horn again. At this point, my instructor senses something not right (operator error or ?) and takes the controls. I'm thinking he's going to do some magic experienced pilot stuff and get us on an even keel, and I can debrief with him later what I was doing wrong. Nope. Stall horn, level off, gain a little air speed, try to climb again, lather, rinse, repeat. We had made it to Parker, which by this time we should have been at 7,500' We were barely at 7,000'. This was not good. At least I was comfortable knowing it wasn't operator error, but that still didn't change the fact that this plane was just not gaining altitude near fast enough to suit our tastes. We contemplated turning back, but weren't quite ready to call it a day. After all, this was supposed to be the fun long cross country flight with my daughter that she would remember forever. (She was oblivious to any difficulties we were having in the front row of seats, continuing her enthusiastic documentation of every minute of the flight on Instagram to rub it into her friends.)

We finally got enough airspeed to where we could climb without the stall horn chirping, but we were not climbing very quickly at all. My instructor checked the density altitude for Colorado Springs. They had a density altitude of 9,200'. Ugh. Not good. If we were getting this poor performance here, we'd likely get even worse performance there. Decision time. The enVironment was turning against us. We weren't losing altitude, so immediate safety wasn't an issue. There were External pressures to continue the flight. My daughter deserved to have a fun, full flight.

I'll be honest in saying I was solidly on the fence at first. I said "let's give it a minute" to see if things calmed down. Again, we weren't losing altitude, so an extra minute to see if things changed wasn't that much of a risk. The stall horn chirping while we were only 500' above the ground was unnerving, but as we gained altitude and began to quiet the stall horn, there was a moment when I thought maybe things were changing. 

Things didn't change. I could sense my instructor was just a bit uncomfortable. This wasn't a "what do you think?" kind of situation where he's asking because he could go either way and wants to know my thoughts. He was genuinely concerned. That was enough for me. That's it, we're landing. Not worth the risk. Live to fly another day. We told ATC we were returning to the airport. They cleared us to land on the runway we just took off from. I made an uneventful approach and despite flaring just a bit too early (now the plane wants to fly!) sat down safely and just a little bit past where I was hoping to. We taxied back to the ramp, tied down, took a few more pictures, and headed in to try to figure out why there was such a discrepancy between what was on paper and what we experienced.

We pulled up the performance charts, and "on paper," we should have had been able to achieve a climb rate near 350 feet per minute. In the 10 minutes from when our wheels left the ground to when we decided to land, we gained only 1,680' in elevation. That's 168 feet per minute, about half of what we were expecting. It's important to note that the "service ceiling" for any aircraft is the altitude at which its climb rate falls below 100 feet per minute. We weren't too far above that at all. Suffice to say even if we were to continue the flight, I would have spent the time worrying about the poor performance, not concentrating on flying the route.

Factors? First, there's obviously the weather. Planes don't perform well at high density altitudes, and performance charts may not mirror real world conditions. How much that is off is hard to measure; it's just something you have to feel in the air. Second, there's the age of the plane. Compared to some I've flown, this plane is a relative Spring chicken. But age still plays a factor. What we think played a large part is the spinny thing at the front of the plane.

Like any mechanical device, you can get speed or power, but getting both at the same time is a rare treat. Airplane propellers work by creating thrust as the blades spin around. Depending on the pitch of the propeller, you can get good speed or good power. Complex airplanes have variable pitch propellers, which allow the pilot to set the pitch of the propellers depending on need. The trainers don't have that feature. A propeller can either be weighted towards good climbing performance--a climbing propeller, or good cruise performance--a cruise propeller. Our plane today was fitted with a cruise propeller. Because of that, its performance on climbs will be less than what's in the manual. This wasn't something I really thought about. I had flown another Archer with a cruise propeller a few lessons back, but didn't notice any real performance issues with it. But, it was a newer plane and we were carrying less weight on that flight. Discussions with other instructors after we landed revealed this plane is known to be a poor performer in hot weather. Now we know.

Takeaway from the day? If something doesn't feel right, get back on the ground. We could have proceeded and likely would have completed the mission. But the risk--especially with passengers--is not worth it. That and I'd have spent the entire flight worrying about the performance instead of enjoying the fight. (To say nothing of the fact if any ensuing crash didn't kill me, my wife most assuredly would have.)