Proper piston single (non-turbo) decent techniques

[KS Arrow Pilot]

I don't fly a turbo-charged airplane, but I believe you need to maintain a relatively slow decent with power so that the engine doesn't cool too fast and possibly crack the turbo unit. That got me thinking - is there a "right" way to descend a non-turbo'd engine so that you don't harm it as well? Maybe I learned it 20 years ago when I got my license, but i've forgotten the right technique. I'm VFR only, and I like to throttle back to 18 or 20 inches in my Arrow, which keeps the knots at 130 with a 300/400-ish FPM decent rate. I watch the oil temp, and it usually stays about the same throughout the decent.

I just had my engine rebuilt, so I want to make sure I'm treating it right as it breaks in and hopefully runs well for a very long time. Thanks

 

[gismo]

I don't fly a turbo-charged airplane, but I believe you need to maintain a relatively slow decent with power so that the engine doesn't cool too fast and possibly crack the turbo unit. That got me thinking - is there a "right" way to descend a non-turbo'd engine so that you don't harm it as well? Maybe I learned it 20 years ago when I got my license, but i've forgotten the right technique. I'm VFR only, and I like to throttle back to 18 or 20 inches in my Arrow, which keeps the knots at 130 with a 300/400-ish FPM decent rate. I watch the oil temp, and it usually stays about the same throughout the decent.

I just had my engine rebuilt, so I want to make sure I'm treating it right as it breaks in and hopefully runs well for a very long time. Thanks

Instrumented studies have shown that "shock cooling" piston engines is pretty much an OWT and while I wouldn't recommend regularly closing the throttle and diving at redline, almost anything that results in 500-1000 fpm at speeds ranging from Va to Vno won't harm anything. Certainly the notion that one shouldn't reduce MP by more than 1" per minute that some have recommended is overkill to the nth degree.

There is an exception for some if not all geared engines and radials that involves never letting the prop drive the engine but neither of those issues apply to a direct drive horizontally opposed engine as found in most GA airplanes.

 

[Sac Arrow]

Just don't worry about it. I eventually gave up the practice of stage cooling my Turbo Arrow. Nothing bad happened to the engine.

 

[BillTIZ]

I fly a 250HP O-540 fixed pitch. Full power climbs, at Vx and VY leaned to max rpm for 6 to 8 min climbs, (~3000 ft AGL). Push over and pull the power back to about 2200 rpm and push up to 80-100kias and up to 1000fpm descents. The prop is unloaded, further reductions to 2000 and 1700 rpm by downwind and 1500 on final, speed down to 70-65 and decreasing. No CHT or EGT available, just oil temp and pressure. No engine problems.

 

[Henning]

Your Arrow, do what you want, your engine won't be worse off for it.

The primary factor that I use in determining rate of descent is my passengers ears.

 

[gismo]

Your Arrow, do what you want, your engine won't be worse off for it.

The primary factor that I use in determining rate of descent is my passengers ears.

So when you fly solo, you point the 310 straight down? Or do you have a secondary factor?

 

[Henning]

So when you fly solo, you point the 310 straight down? Or do you have a secondary factor?

Straight down is good with me, I'm a long time deep diver, I can clear swimming for the bottom at 240' by just exhaling out my nose on the way down. A lousy 1/3rd of an ATM is nothing.

 

[gismo]

Straight down is good with me, I'm a long time deep diver, I can clear swimming for the bottom at 240' by just exhaling out my nose on the way down. A lousy 1/3rd of an ATM is nothing.

I've never had any trouble clearing my ears (flying or diving) but I do seem to suffer some sinus issues on a long fast descent. In any case the airplane and it's engines might have an issue with a significant vertical leg.

 

[Henning]

I've never had any trouble clearing my ears (flying or diving) but I do seem to suffer some sinus issues on a long fast descent. In any case the airplane and it's engines might have an issue with a significant vertical leg.

The air frame may have issues eventually, but not the engines, not on a cooling issue anyway.

 

[Sac Arrow]

The air frame may have issues eventually, but not the engines, not on a cooling issue anyway.

Airframe? As if you whack it in to the ground or overstress it in Vne?

 

[Henning]

Airframe? As if you whack it in to the ground or overstress it in Vne?

Take your pick, the last leads to the first anyway, either way it'll be cool on You Tube.

 

[hindsight2020]

chop and drop baby.

 

[Henning]

There is not a plane I've flown that if taken from a power off stall you couldn't push through holding approximately 0g all the way to nose down vertical, hold a solid 2 count (more if you fly a draggy biplane) if not more until you get to 2/3rds green arc. If at that point you initiate a 3g pull you'll do this with a minimum loss of altitude all well within the 3.8g performance limits of any GA airplane. Most recip planes being flown here it'll be less than 1000' loss from initial entry altitude to recovered.

That won't be a reality for most pilots without an accelerometer display or at least a good working knowledge and use of a 6g+ airplane. Most people do not believe the first time they pull 3g that it's only 3g. In this day and age where 60* for a steep bank is considered either dangerous or too difficult, it's quite possible for a pilot to fly his entire life and never pull 2g. I find most people will quit pulling at 2.2- 2.5g out of fear if not stark terror; 'there is no way in hell this biscuit tin can take this!' was the way my friend Janet put it when I demonstrated a level 60* bank in a 152.

The good thing is, even pulling just 2g in the recovery will keep you well withing green arc energy limits.

My advice if you find yourself looking straight at the ground with the speed climbing is pull till it scares you to keep the airspeed in the green arc. If the airspeed keeps climbing, pull harder. Your goal in this recovery is to pull hard enough to keep the ASI in the top 1/3rd of the green arc. This keeps you in a structurally safe region while giving you enough energy to help prevent accelerated stall issues in the recovery.

I highly doubt anyone without high g training and experience will ever pull 3.8g

 

[poadeleted20]

Instrumented studies have shown that "shock cooling" piston engines is pretty much an OWT

If you're speaking of quench cracking, yes, that's true. If you're speaking of accelerated wear, Lycoming still says you can get uneven cooling of the heads and pistons if you chill the heads too fast (60-100 deg/minute), and that can reduce cylinder life. We're not talking about catastrophic failure here, just the difference between, say 2000 hours and 1600 hours before overhaul is needed.

and while I wouldn't recommend regularly closing the throttle and diving at redline, almost anything that results in 500-1000 fpm at speeds ranging from Va to Vno won't harm anything. Certainly the notion that one shouldn't reduce MP by more than 1" per minute that some have recommended is overkill to the nth degree.

Agreed, and if you have a good engine analyzer, you can see what will cause cooling rates of more than 60 deg/min.

 

[gismo]

The air frame may have issues eventually, but not the engines, not on a cooling issue anyway.

Not cooling but there's some chance you'd end up with an overspeed about the time the wings came off unless you feathered the props.

 

[gismo]

There is not a plane I've flown that if taken from a power off stall you couldn't push through holding approximately 0g all the way to nose down vertical, hold a solid 2 count (more if you fly a draggy biplane) if not more until you get to 2/3rds green arc. If at that point you initiate a 3g pull you'll do this with a minimum loss of altitude all well within the 3.8g performance limits of any GA airplane. Most recip planes being flown here it'll be less than 1000' loss from initial entry altitude to recovered.

That won't be a reality for most pilots without an accelerometer display or at least a good working knowledge and use of a 6g+ airplane.

While I agree that under normal circumstances most pilot won't pull more than 2-2.5g if that were entirely true we wouldn't have reports of pilots pulling the wings off perfectly good airplanes. A windshield full of ground coming up fast might provide the incentive to pull harder than ever before.

I highly doubt anyone without high g training and experience will ever pull 3.8g

I do know from personal experience that when seated upright I start to grey out at about 5g no matter how hard I tighten my legs and chest.

 

[Jaybird180]

I recall a NTSB report (?) about a Bonanza (?) where the pilot nearly lost it and caused severe stress cracks in the recovery. Landed safely, then the airplane needed significant repairs.

 

[Jay Honeck]

I highly doubt anyone without high g training and experience will ever pull 3.8g

Agree. I pulled 4.5 Gs in a Decathlon, and it felt like a hundred. I have never pulled half that many in Atlas, nor would I want to.

 

[Oldman]

Agree. I pulled 4.5 Gs in a Decathlon, and it felt like a hundred. I have never pulled half that many in Atlas, nor would I want to.

Yus. 4 Gs and you say, "What, that was only 4 Gs?? Don't lie to me!"

Still, pulling Gs in a GA aircraft with the assured comfort that probably you aren't exceeding the structural limits doesn't seem wise.

- jkw

 

[Henning]

While I agree that under normal circumstances most pilot won't pull more than 2-2.5g if that were entirely true we wouldn't have reports of pilots pulling the wings off perfectly good airplanes.

I think that is far more related to the opposite side of the same coin: They aren't pulling enough Gs therefore aren't scrubbing enough energy and they run into Vne issues. That's why it's important not to be afraid to pull, because when the nose is down, pulling is what keeps speed down.

 

[flyingron]

The engine analyzer starts blinking if it thinks you're descending too fast. I've never really had that problem. Normal enroute descents I don't normally touch the power. Descending into the approach, I usually pull back to 20" because most often I also need to slow down.

 

[DouglasBader]

I flew turbocharged piston airplanes in an area for one particular employer, which required power off steep descents. The employer taught these at night, with tight high traffic pattern and a blacked-out idle descent to landing. I wasn't comfortable with treating the engines that way at first, but found that the engines used in the fleet (about 30 aircraft) all made TBO without any difficulty.

On the other hand, I flew normally aspirated and piston airplanes for another operator with roughly the same number of aircraft in the fleet, which experienced lifted or cracked cylinder heads and engines that didn't make it to TBO. The second fleet used bigger bore motors and had a wider range of pilot experience (and probably technique, too).

I'm a big proponent of treating the engine as though your life depends on it (it often does). That includes descents in single engine normally aspirated piston airplanes. Most of the time you can accept a higher airspeed, and make a slight (if any) power reduction. If it's turbulent, reduce more power, slow down, descend a little slower, as needed.

If you're unpressurized (pretty much a given if you're in a normally aspirated aircraft), the descent rate should be a comfortable one, at a thousand feet a minute or so. Most descent profiles match about a 3:1 descent: start three miles out for every thousand feet you're going to descend. Calculate your ground speed and descent time to do it, and adjust your power accordingly.

If you're flying a 120 mph airplane, you're covering 2 miles a minute. If you're flying at 10,000', you've got 30 miles to make a 3:1 descent. that's fifteen minutes flying, which only about 700 fpm; you don't need to reduce the power much do to that. If you want to do a 2:1 descent, then that's 20 miles, or ten minutes away. that's a thousand feet a minute, which is about right for a light airplane descent. Adjust the power to keep your speed where you want it at that rate, and you're doing okay. Tweak your mixture up a hair at a time.

If you happen to be flying where there are mountains (which is most places I fly), then your descent will vary with the terrain and destination. Plan and adjust accordingly.