Aviation Safety Corner
this month's topic, I would like to continue our discussion on, "Initial Climb Strategies." Exploring
the pros and cons of using your airplane's best-rate versus best-angle climb speed after liftoff. This is Part
III of IV.
Now, let's discuss, "Cutting the Margin." You were getting close to stalling and, maybe, even going down before you heard the stall horn or felt any buffet. That's what you are asking for when you try to fly a Vx climb after takeoff!
To me, a Vx climb at low altitude is akin to setting up for a departure stall right after takeoff. Sure, if you establish just the right pitch attitude for your field elevation and OAT (density altitude), maintain the published Vx, don't get any wind gusts and don't get any side slip angle because you are holding sufficient right rudder, you will have a slightly better climb angle than you would at Vy.
But just get a few mph or knots slower than Vx, and you may have zero climb rate, since you are on the back side of that thrust curve, and induced drag is going sky-high. When this happens, even if you don't really stall and spin, you are about to become a stall/mush accident statistic.
Consider this: What would you think if your CFI was happy with your power-on and power-off stall demos at 3,000-4,000 feet, then suggested that he wanted to see a takeoff stall, approach only, right after liftoff?
No, he doesn't want a stall; he just wants to see you lift off, start a climb, then keep hauling the nose up until the stall horn starts sounding or you feel the first buffet. Of course, you would throw the CFI out of the airplane and look for someone a little smarter.
But this is what you're playing with when you try to fly at Vx right after liftoff. That Vx airspeed is part way down the approach to a power-on stall. And it probably is very close to the power-off stall speed, should you suddenly lose engine power. In a multiengine airplane, Vx and Vxse may be perilously close to Vmc (minimum single-engine control speed) or engine-out stall speed.
You multiengine drivers should remember that if you lose one engine just a little above a two-engine, power-on stall speed, the wing on the dead-engine side may be at or below its power-off stall speed.
Maybe that is why the flight manuals for many multi-engine airplanes do not show a Vxse climb speed. That speed is just too close to both Vmc and engine-out stall airspeed. Again, in my thinking, an initial Vx climb, whether in a single or a twin is akin to slow-flight or approach-to-stall practice, and too close to the ground.
Now, let's discuss, "Trees in the Window." Is there a real use for a Vx climb? Well, yes, there is-if you should ever find yourself facing trees, houses, power lines, etc., directly ahead shortly after liftoff, and there isn't sufficient room to land straight ahead.
Maybe the wind shifted. Maybe the engine isn't putting out full power. Maybe you didn't calculate Hd (density altitude) and make applicable correction to the figure you got from the takeoff distance chart in your AFM. Maybe that soft or rough airstrip was bumpier and "draggier" than you thought it was. Maybe you didn't run the weight/c.g. numbers, and your airplane is loaded a little too heavy. Maybe you are not quite as sharp as the airplane manufacturer's test pilot and engineer who calculated the AFM climb data.
So, yes, there may be a time when you need to grab as much altitude in as little distance as possible. But, before you ever try a Vx climb right after takeoff, you should practice way up high, at several thousand feet. You should practice the rotation rate for lifting off into a Vx climb. You should memorize the visual picture for the pitch attitude needed to hold Vx at a density altitude similar to what you expect to have tomorrow afternoon when you depart from the "Mount Baldy" wilderness strip.
You also should consider that if you had trouble this morning landing and stopping at Mount Baldy, you might have even more trouble departing from this short strip that's surrounded with obstructions.
Most of our General Aviation airplanes can successfully land and stop at strips shorter than required for a safe takeoff. Yes, most GA airplanes can glide, in landing configuration, at an angle that's steeper than what they can achieve during a climb in takeoff configuration. And, with good braking, they can roll to a landing stop in a shorter distance than their accelerate-to-liftoff takeoff distance.
Maybe we shouldn't put ourselves in a situation in which a Vx climb after takeoff is needed. But if we do find ourselves in such a situation, we should consider off-loading our passengers and cargo, and attempt that takeoff and initial climb by ourselves, recognizing that performance is only going to be worse with all the people and bags aboard.
In conclusion, remember a Vx climb should be considered a do-it-right-or-die maneuver. It should not be done intentionally with passengers aboard.
Don't miss next month's Aviation Safety Corner. I'll finish our discussion on, "Trust the Table?" and "Snapping Point." This will be Part IV of IV.
Larry G. Harmon
FAA AVIATION SAFETY COUNSELOR
for Bill's March Message
for our March Flying Journal
to return to our Main Page
to return to the MPA Home Page
to visit EAA Chapter 821's Web Site