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Aviation Safety

AVIATION SAFETY

Dave Fry, WVFC CFI and Aviation Safety Counselor dgfry@aol.com

Stall!  Stall!  Stall!

One of the Pilatuses I fly has a British female voice that, when the speed gets too low, calls (loudly enough for the passengers to hear) Stall! Stall! Stall! Parenthetically, she’s referred to as Bitching Betty.

We rarely think about stalls except during training flights, but we get pretty close to stalling on most flights, even when we don’t go out specifically to practice them.  That would be the landing, of course, but sometimes we do them intentionally to build both stall recognition skills, and to hone our recovery techniques.  And sometimes we get really close to stalls when we don’t want to.  In that spirit let’s look at stalls (and recoveries), in a couple of different ways: how do they happen when we don’t want them to, and when do we recover from the stall both for the PTS and in the real world.

Before diving into that, I’d like you to ponder the following question: If the bottom of the white arc is the full flaps stall speed (and it is), why can we do slow flight five knots below the bottom of that arc?  And we can, so the question isn’t theoretical; it’s real and relevant.

So we all know that stalls occur when the angle of attack gets great enough that the air flow over the wing doesn’t provide enough lift, and with a bit of work, we can probably remember things like relative wind and cord lines.  And we may even remember our instructors’ admonition that a stall can occur at any attitude and airspeed.  However, in order to occur at a high airspeed and a nose down attitude the G forces have to be really high.  And in that case, things get really exciting in a hurry.  Still, unless you’re SERIOUSLY over gross, or involved in some form or aerobatics, it’s really hard to stall with the nose below the horizon, or at normal cruise speeds, or at full power with the nose at or below the normal takeoff attitude.

Stalls occur unintentionally when the pitch increases, when the load factor goes up, or when the speed drops unexpectedly.  Good academic knowledge, but what does that mean to the pilot, and how do those things occur unexpectedly?  Like Murphy’s Law, which states that if anything can possibly go wrong, it will (and at the worst possible time), stalls often occur at the worst possible time, and that means when you’re slow and near the ground, as on the base to final turn, for example.  Typical scenarios include banking too steep to keep from flying through final, which increases the load factor and raises the stall speed up to what you were flying before starting the turn.  Or raising the nose because your glide path looks like it may end up in the water rather than on the runway, which, of course, means you slow down and stall.  Or you are right on the approach speed and a wind shear drops your airspeed by ten knots and the stall warning goes into overdrive.  Intentional stall causes can be even more varied, but usually involve a flight instructor.

The point is that unintentional stalls usually result from technique issues, or failure to anticipate conditions properly.  In other words, most stalls can be avoided.  So, if they can be avoided, why do we practice stall recoveries?  Don’t know about you, but I’ve made a mistake or two approaching the runway (to say nothing of other times), and I want my recovery technique to be automatic, and correct.

WHEN do you recover from a stall?  To some extent, it depends – which isn’t a completely satisfactory answer, but allow me to expand on that.  According to the Practical Test Standard, the Private Pilot applicant needs to recover from a full stall, but the Commercial and ATP Practical Test Standard requires recovery at the first indication of a stall.  Guess which one makes more sense for the unintentional stall.  Clearly, recovering at the first indication is in order, whether that indication is a buffet, or a stall warning horn or light. 

And those incipient stall and full stall recoveries aren’t quite the same.  In the incipient stall scenario, the pilot typically keeps the same pitch attitude, but brings in full power, cleaning up as appropriate.  On the other hand, if the stall has already occurred, the nose comes down first, and then full power is added, which leaves the plane pointed at the ground and accelerating – not a good long-term configuration near the ground, but fortuitously most planes will pitch up as power is added.  So we raise the nose (or let the power bring it up) to our normal climb attitude, and start cleaning up the flaps and gear.  However, and this is important, following the POH is critical.  In some cases, like the P-51 Mustang (not that we all get to fly them), going to full power at the indication of a stall will result in a torque roll that can’t be controlled by full aileron.  This, in turn, results in half of a roll and an inverted crash into the ground – Game Over.

Even if you are working on your Private license, I’d recommend practicing both the full stall recovery and the incipient stall recovery.  Actually, I’d recommend it for both Commercial and ATP, as well.

And about that question earlier – how can we fly 5 to 10 knots under the bottom of the white arc without stalling?  The white arc shows the slowest speed at which we can fly with full flaps with the plane at full gross, and in most training situations, we’re dealing with far less than full gross weight, so there is a lower load factor and therefore a lower angle of attack at a given airspeed.   So we can fly slower than the bottom of the white arc.

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