Power + Pitch = Stall (?)

Kris Johnson has a posting on holy wars in aviation, including the two variants of the very dangerous teaching that you control airspeed with pitch and power. The idea is that students learn to look out the window (which is good) and, for any given RPM — assuming a fixed-pitch prop — memorize what pitch attitude will give them what airspeed. I believe that this teaching approach has two significant consequences:

  1. it gets student pilots to first solo faster; and
  2. it kills some of those pilots (and their friends and families) after they get their licenses.

While we all want to save money on flying lessons, I think this is a lousy tradeoff. The only way to make a given pitch/RPM combination produce a given airspeed is to make sure that gross weight, density altitude, CG, sideslip, and bank are always exactly the same. During training, while these conditions are not always exactly the same, they’re usually pretty close, so instructors can get away with this little cheat, students solo sooner, and everyone’s happy.

Departure Stall

Some day soon, though, the new private pilot is going to want to do more than fly around the same area under the same conditions with zero-to-one passengers on board, and unfortunately, he or she is going to be in for a big surprise. Consider this: after training through the fall, winter, and spring, the pilot (we’ll make it a male) finally has his PPL, just in time for summer vacation. He loads his wife and two children on board a Cessna 172 or Cherokee, secures the baggage in the back, and fills the tanks (making sure to stay withing W&B, of course). It’s a bit of a hot, humid day, but it’s a low elevation airport and a nice long runway, and the pilot must have taken off here 50 times during training, so it shouldn’t be a problem.

It takes the plane a lot longer to get up to rotation speed in the takeoff roll, but the pilot expected that — that’s why they have those takeoff-distance charts in the POH. It’s also much harder to unstick the plane from the runway; in fact, it seems to want to settle right back down again. Finally, though, the plane is climbing … well, sort of. The pilot is used to about 700 fpm with half tanks and an instructor on board, and 1,000 fpm or better when he’s solo; a quick glance at the VSI, though, shows only about 200 fpm. HUH? Well, the pitch is wrong — the nose is too low compared to every other climbout the pilot ever did — so he pulls the nose up a bit to get closer to the normal climb pitch. The VSI jumps up to 500 fpm, so this was obviously the right choice … except that three seconds later, it’s down to less than 100 fpm. The plane isn’t climbing at all, for any practical purpose. OK, pull the nose up a bit more (the tach shows full power, and the nose is still lower than usual), and sure enough, the VSI jumps up a bit more, before it settles down again, this time with no climb at all.

You can see where this is leading — one or two more pullups (still below the normal climb pitch angle from training) and the flight ends up as one of the many, many summer stall-spin accidents on takeoff. According to the Transportation Safety Board of Canada statistics, from 20-33% of accidents take place during takeoff, literally during the first few seconds of flight. In this case, a glance at the ASI should have told the pilot that the nose was too high, not too low, but the problem was that the nose looked too low, because the pitch was too low, and pitch + power = performance. Normally, the pilot could barely see the ground ahead at all in a Vx or even Vy climb, but this time, the ground seemed to fill a third of the windshield. Even if the pilot did look at the ASI, he might have had trouble believing it, since it was outvoted by the pitch (which showed the nose too low), the power (which showed the correct RPM), and the VSI (which showed the climb rate way too low), and all of that was combined with the stress of having the family on board for the first time, etc. etc.

Angle of attack, angle of attack, angle of attack …

Here’s the problem — it’s not pitch and power, but angle of attack that controls airspeed. The pilot studied angle of attack during groundschool, of course, but it never seemed all that practical, and the instructor’s “pitch + power = performance ” mantra seemed simpler and more logical. As long as the plane was flying under similar conditions (density altitude, gross weight, etc.) you could pretty-much map pitch angle to angle of attack during each phase of flight — there was a “climb attitude”, a “cruise attitude”, and an “approach attitude”. In the example above, however, there was a significant change to some of the conditions, so the mapping didn’t work any more.

My Warrior, for example, might climb as fast as at 1,200 fpm on a winter’s day near sea level with just me on board and half fuel; it can barely manage 400 fpm (at Vy) from the same airport with the whole family, dog, baggage, and full fuel on a hot summer day. Since Vy for my Warrior is 80 knots, assuming no wind I’ll fly forward about 8,100 feet every minute — if I’m climbing at 1,200 fpm, I’ll follow an angle of climb of nearly 9 degrees (!!); if I’m climbing at 400 fpm, I’ll follow an angle of climb of about 3 degrees. Now, my angle of attack, which controls my airspeed, goes on top of that. Let’s say that a 4-degree angle of attack gives me Vy (I’m just guessing):

  • when I’m flying light on a cold day, my pitch angle will be 4 degrees for angle of attack, minus (say) 2 degrees for the incidence angle of the wings (which are normally tilted up a bit), plus 9 degrees for the angle of climb, giving me a pitch angle of 11 degrees — I’ll see nothing but clouds and sky outside the windshield.
  • when I’m flying heavy on a hot day, my pitch angle will be 4 degrees for angle of attack, minus 2 degrees for the incidence angle of the wings, plus 3 degrees for the angle of climb, giving me a pitch angle of 5 degrees — the bottom third of the windshield will be filled with the ground.

In one case, a pitch of 13 degrees + full power = 80 knots; in the other case, a pitch of 5 degrees + full power = 80 knots. Different pitch, but same airspeed and same power. Clearly pitch + power != performance.

Why do some instructors do this?

Of course, there are many instructors who know the dangers of the pitch + power thing and teach it properly: you have to get to the right airspeed first, and then choose the pitch angle that gives you that airspeed at that power setting at that particular moment — it might be different every time. And you have to choose a new pitch angle not only for every flight, but for every power change in that flight (and even for the same power setting over a long trip, as you burn off fuel and get lighter). Taking the shortcut — memorizing preset pitch angles — might get you to first solo faster, but it might also kill you.

Part of the problem with the instructors who get it wrong might be the fact that a lot of them — especially the ones using flight instruction as a career step — have a surprisingly limited exposure to aviation. It’s even possible that some of the ones who are building time to get into the airlines might never have gone on a really long cross country (the one for the CPL is only a few hundred miles), never have flown a low-powered plane near gross weight on a hot day, never have taken off from a short, obstructed grass strip, never have flown an approach in actual low IMC, never have tried to maintain control inside a TCU with lightning flashing around them, never have done a continuous 180 descent from close downwind to landing to avoid holding up three jets on final at a busy airport, etc. etc. Their experience doesn’t really match much that their PPL students will be doing if they choose to remain private pilots, so instead, they pass on platitudes and old chestnuts that they learned from their instructors, and spend a lot of time on relatively pointless exercises designed to get the student past the flight test rather than helping him or her become a safe pilot.

About these ads

About David Megginson

Scholar, tech guy, Canuck, open-source/data/information zealot, urban pedestrian, language geek, tea drinker, pater familias, red tory, amateur musician, private pilot.
This entry was posted in Uncategorized and tagged , , . Bookmark the permalink.

7 Responses to Power + Pitch = Stall (?)

  1. Fred W. says:

    Brilliant explanation! I’ll be reading you regularly from now on.

  2. Joel Jacobsen says:

    You’re saying that pitch doesn’t control airspeed, but then you say to choose the pitch angle that will give you the proper airspeed for the power setting. It sounds like you just contradicted yourself.

  3. david says:

    Thanks for the comment, Joel. Memorizing a standard pitch angle for a specific airspeed at a specific power setting works only when weight and density altitude are roughly constant. Unfortunately, while they tend to be (close to) constant during training, one of the first flights a person takes after a PPL typically involves loading up the plane with buddies or family, and suddenly the old, memorized pitch angles become very dangerous, and we start to see those fatal departure-stall accidents (especially in the summer). You’ll still have to use pitch angle in any given flight to avoid chasing the ASI, but you cannot safely use a pitch angle that you’ve memorized in advance — you need to establish a new pitch angle every time you fly.

  4. Chris says:

    This is taught by instructors who have been taught with a view to fly an airliner with an abundance of power. It’s not safe in lower powered aircraft (i.e. for all the rest of us). When students learn to fly VFR with a single engine plane they do not need an AI. In fact, they should be taught to fly for airspeed when climbing, that’s all that matters. As you point out, the pitch follows automatically.

  5. harry freeman says:

    good explanation of stall, but seems too complicated…what if pilot just flies the climb airspeed and just settles for whatever climb rate he gets for conditions of the day? (to heck with all that pitch stuff) freeman@midmaine.com

  6. david says:

    Airspeed lags a big after pitch changes, so it’s easy to end up chasing the needle (unless you’re very good at damping out oscillations).

  7. Pingback: Land and Hold Short » Disjointed notes on my GaspĂ© trip

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s