Menu Sign In Contact FAQ
Banner
Welcome to our forums

Closing the throttle in flight - nicely (shock cooling)

Elsewhere, a discussion included power off flight. I reminded myself, while winter flying yesterday, that considerations of closing the throttle in flight in air cooled engine powered planes are sometimes not clearly understood.

Presuming that closing the throttle is voluntary, and not at all rushed, as you applied your excellent pilot planning to not have to suddenly descend, close the throttle very gradually. When I used to fly the turbo Aztec, the drill was 2" of manifold pressure reduction per minute from cruise flight in winter. For a slower, lower power normally aspirated plane, it’s not that critical, but still go easy. If you have them, close the cowl flaps first, and if you can, slow the plane down somewhat first.

The reason for this added care is your eager effort to not shock cool the engine. Consider an air cooled engine happily developing 75% power on a cold winter day in cruise flight – it’s being air cooled with cold winter air, exactly as designed, it’s making heat, which is being taken away exactly as designed, and the whole engine is in it’s nice temperature equilibrium as designed. The cylinder heads (which are very air cooled) are properly expanded.

Then, careless pilot snaps the throttle closed. The cooling continues, just as it had been the moment before (‘cause the air temperature and airspeed did not change), but all that heat suddenly reduced a lot. When you suddenly cool metal, it shrinks. Cylinder heads are uneven in arrangement, so they shrink unevenly. If that happens too fast, they crack. When I worked for the engine shop, I removed a lot of cylinders to find a crack or two in them. The cracks are usually hard to see from the outside, but when you clean the inside, they’re usually easy to see.

The pilot won’t really notice that this has happened while flying, but the next person who measures the engine power, or do a compression test will certainly notice. That cylinder is possibly ruined, but at the very least will have to be removed for repair/replacement. This is totally a “pilot broke it” or careful pilot did not break it" situation, there is no half way – it happened in a matter of seconds, and is done.

This situation is entirely unrelated to “warming the engine during a practice glide”. Sure, do that, but if you’ve already snapped the throttle closed, the damage has been done. The engine will still get you home, but you’ve caused costly damage.

This kind of care while flying is what the experienced owners of airplane notice. When they see you caring for the lane you fly, and understanding how to reduce engine wear and damage, they are much more inclined to allow you to fly their plane. This will be the entry point to gaining experience on more exotic types – being known as a pilot who cares about the plane…

Home runway, in central Ontario, Canada, Canada

So on a hot day, you fly into a rain squall. The damage must be immediate due to the rapid cooling. I am not a believer in shock cooling.

KUZA, United States

NCYankee wrote:

So on a hot day, you fly into a rain squall. The damage must be immediate due to the rapid cooling. I am not a believer in shock cooling.

I am also between skeptical & not believer still like to plan well along 2"/min if my beliefs turn out to be wrong but I have seen CHTs it going from 380F to 240F quickly, I am sure the curve was even steep somewhere along 50F per few seconds (butthe CHT probe was slow to catch it), that gap has nothing to do with 240F to 32F in 10 seconds after shutdown, so it was hard on the engine

Again not sure how much is good practice vs actual concern (one will be surprised by the funky shape of cylinders in both well managed as in slammed engines) but everybody should agree that going slowly will not do any harm

Last Edited by Ibra at 24 Feb 13:57
Paris/Essex, France/UK, United Kingdom
always learning
LO__, Austria

I’ve read that the difference between 400°F to 240°F quickly and 240°F to 32°F quickly is that at 240°F, the metal is harder than it is at 400°F.

ELLX

Surely in that case the training fleet should be ruining cylinders constantly, and need cylinder changes all the time, if doing this once is so likely to ruin the cylinders? Yet the training fleet often seems to manage TBO. This included for example the Geronimo Apache that I did my multi in (which regularly had an engine actually shut down completely in flight, and done in a way to surprise the student).

What about the other way around – “shock heating”? My own aircraft is used to tow gliders and the positive rate of change is very high, and repeated multiple times every time we tow (with very short 10 minute flights). Yet the cylinders always have excellent compression on the annual check.

Andreas IOM

lionel wrote:

I’ve read that the difference between 400°F to 240°F quickly and 240°F to 32°F quickly is that at 240°F, the metal is harder than it is at 400°F.

Yes profile for thermal/mechanical elasticity are not linear and surely at 400F thing are different !

The general rule for metal fatigue is to keep temperature loads frequency/amplitude in sync or way bellow mechanical loads, the case of fast cooling on high engine power would make 100h TBO (or any case of non-sync thermal & mechanical loads) but I doubt this ever happen while flying, ditching in Alaska ice lakes?

Usually in piston aircraft the mechanical loads have fast cycles that is almost applied on constant temperatures, otherwise high school thermodynamics will be messy (this is not the case in hot turbine chambers, where the whole thing can melt before even the compressor can make one revolution)

Last Edited by Ibra at 24 Feb 14:38
Paris/Essex, France/UK, United Kingdom

Shock cooling search

The general reasoning is that the metal starts to lose its strength above say 350F CHT, and above that the loss is rapid especially past 400F. This addresses the usual objections to the idea of shock cooling

  • the training fleet (together with many private owners) flies mostly full-rich and is not reaching anywhere near 350F
  • the issue doesn’t exist during departure because the CHT takes plenty long enough to build up
  • schools do get cracked cylinders
  • the CHT is engine dependent, with the common O-200 running pretty cool (often “too cool” to get the oil warm, leading to the fitting of air deflectors)

The rain related objection is addressed by considering the latent heat of evaporation of water and the mass flow of water entering, versus the specific heat capacity of air and its mass flow at 100kt+. Even really heavy rain involves very little water, as a % of the volume of the space traversed by the cowling inlet. The supporting evidence for this is watching the CHTs during flight in heavy rain; they hardly change.

Administrator
Shoreham EGKA, United Kingdom

What is a good way to deal with this issue during flight training where you often want/need to quickly apply or reduce power?

ESSZ, Sweden

I think ATOs are famous for flying all day on full rich on low 60% power setting, so their CHT will always be in the 280F-300F region?
Maybe only a concern for someone doing aggressive LOP on 75% power and going for 130kts decent high ROP on low power

Last Edited by Ibra at 24 Feb 16:58
Paris/Essex, France/UK, United Kingdom
88 Posts
Sign in to add your message

Back to Top