Thursday, August 15, 2013

Standard Approach Profile

To land a tricycle gear airplane on a runway is not very difficult. You just place it in the correct slightly nose up attitude and keep it straight while the main gear settle onto the runway, and then you gently lower the nose. The difficult part is to arrive at the beginning of the runway at a speed such that when you raise the nose the appropriate amount the airplane neither stalls abruptly nor balloons into the air, and to arrive there at an altitude such that the airplane contacts it at just the same moment the wings stop flying. Also the airplane needs to be configured for landing. At minimum, landing configuration should include wheels.

My airplane has a maximum speed at which I can extend the landing gear and three different maximum speeds for extension of different amounts of flaps. In a normal landing I should have the gear and all the flaps down, the final stage of flaps coming perhaps a couple of hundred feet above the runway. In order to have the speed required for that, I aim to be at the speed for the second stage of flaps by 1000' above touchdown, and that's also a good point for me to have the gear down. In order to reach gear speed it helps to have the first stage of flaps extended, usually five miles out, or mid downwind. Until then I can fly any speed I want, so long as I stay below 200 kts in the control zone, or slower if the control zone has its own speed limit. Plus I need to be out of the yellow arc if there is a risk of turbulence. Let's call that by 10,000'. And so on, continuing to work backwards until I'm leaving FL190 and deciding whether to push the nose down for a drag-increasing 2000' fpm descent or leave it gently trimmed for a 400 fpm let down. You learn in initial flight training about the relationship between airspeed and drag: the latter increases with the square of the former, so if I want to arrive at a point in space with less energy, both gravitational potential (i.e. altitude) and kinetic (i.e. airspeed) I need to push my nose down and go faster.

I'm landing today at a larger international airport, but they only have one of their runways open: construction, FOD cleanup or something, so everyone including me is heading for the same runway. I'm asked to maintain 160 knots. Not a problem on the descent, but as I get closer I want to bring the power back on my engines to cool them gradually and I ask the controller if he still needs the speed. He most emphatically does. I have to increase power to hold it. And I'm also above my maximum gear and flap extension speeds.

Remember that the aircraft is designed to fly efficiently through the air. Even if I slam the throttles to idle, which I'm not going to do, because I have to depend on these cylinders not cracking from shock cooling, the airplane will take time and space to slow down. Getting flaps and gear down adds drag, but I have to slow down to be allowed to do that. I'm finally permitted to slow down, so I pull the nose up to slow down until I have approach flap speed. This makes me climb, but they don't mind that. As soon as I add approach flaps that increases drag, and increases the amount of nose down that gives me the same speed, so I can put the nose down while still slowing down and as soon as I have gear down goes the gear, still slowing so I can add more flaps, put the props ahead and on the runway nice and slow, so I can exit at the first available taxiway.

Phew, not how the flight instructor taught it, but still using what I learned (and taught) back in flight school.


Frank Van Haste said...

Good morning, 'Trix!

Let me play Devil's Advocate for a bit, and take the position that "shock cooling" is a myth. But, like all myths, it has its origins in the real world and so contains a grain (but just a grain) of truth. Here's what the esteemed Walt Atkinson has to say:

It is our contention that shock cooling is a myth. I routinely reduce power significantly and never worry about shock cooling the engine. How can you shock cool something that's not hot already? If my CHTs ar eunder 380 or so, I can't shock cool them.

This is an interesting Old Wive's Tale that will not die. Prior to the introduction of the turbo Twin Cessnas, there is no reference to the phenomenon of Shock cooling anywhere in any of the aviation literature. So how did it all get started? In the 421, pilots were descending from the flight levels to land. The pressurized aircraft could descend quickly and not be an ear-popper. The fuel in the tanks was very cold from being at altitude. They'd reduce the power and when they got to pattern altitude, the common practice was to go full rich. This blast of very cold fuel on the inside wall of the warm intake chamber caused cracking. That's where all of this shock cooling stuff came from. The cylinders were not having trouble at all. The problem was the super-cooled fuel from flying high hitting the warm metal.

Shock cooling is a myth for operational purposes unless the CHT is near redline and you chop the power and go full rich. That stupid pilot trick can cause problems.

Walter Atkinson
Advanced Pilot Seminars

Seems to me that if shock cooling were a real hazard then the engine every aerial application airplane and every jump airplane would die of its effects long before TBO...and they don't!

Thanks for the nice post. Taught me something (descend fast to go slow, descend slowly to go fast).

Best regards,


Anonymous said...

And what about slipping ? Although it may be poor form, it allows the airplane to slow without flaps ( or their limitations ) ..

Sarah said...

Frank, I think I've replied to you before on this topic. "Shock cooling" is a real thing - in extreme cases. You say that if it were real, "every aerial application airplane and every jump airplane would die of its effects long before TBO...and they don't!"

I can tell you that glider towplanes do die long before TBO if mistreated this way. Well - we've replaced several O-320 cylinders anyway, before we wised up and installed a digital per-cylinder CHT gauge.

A good CHT gauge will shows that one or more cylinders is running hot - due to full throttle slow climb speed - and is vulnerable to shock cooling if you pull the throttle after the glider release. We now require a more gradual letdown/cooldown especially if things have gotten "warm" up front.

Damage may be as much from running too hot as cooling too fast ... but whatever the reason, it's a temperature change related causal factor to cracked cylinders. QED.

Cedarglen said...

Great post! It sounds an awful lot like knowing your airplane very well, planning well and always staying well ahead of the beast and perhaps even trying to anticipate what the nice folks at ATC will want next. Hell, it is only and airplane!

majroj said...

I'm obviously not a pilot, but do common altitudes (not Mt Everest) slew or skew your speed/altitude/flap-gear considerations? Or for that matter very cold or hot temperatures?

Aviatrix said...

While density altitude (a combination of temperature and altitude) do affect the behaviour of the airplane on approach they do not change the indicated airspeed at which I can extend gear and flaps nor the indicated airspaeed which I wish to maintain on approach. In gusty winds I might choose to approach at a slightly higher airspeed.

Aviatrix said...

I slip sometimes, after briefing on what I'm doing, but an extended slip at the level of fuel in my tanks at the point I choose to land could put me at risk. And it's not very comfortable. It's better to plan the whole descent a little less crazily.

majroj said...

Having experienced some significant slip as a passenger out of contact with the pilot (e.g. in a C-130 cargo bay), I thank you on behalf of all us passengers.