Slowing down for approach without risk of shock cooling and fast enough?

Discussion in 'Cleared for the Approach' started by JasonM, Jun 29, 2014.

  1. JasonM

    JasonM Pattern Altitude

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    I fly a Turbo 182 and was told to decrease my manifold pressure 2" at a time every 2 minutes or so to keep from shock cooling the engine. In my IFR training so far, I am told I need to keep my speeds up until the FAF.

    How do I reduce the manifold pressure slowly to avoid this while complying with my CFII's request to keep my speed up?

    Also I have had a couple approaches where the FAF to the runway seems so close and by the time I slow down, put in flaps and establish the 90 Knots, I am way to high and super close to the runway. Seems to me like I should be able to start setting up for the approach speed at the IAF.
     
  2. Fearless Tower

    Fearless Tower Touchdown! Greaser!

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    I don't have a lot if experience with turbos, so I will ask this: what is your typical cruise MP?

    In my non-T Baron with IO-470s, I find that simply reducing the throttles to maintain my cruise MP setting works well.
     
  3. AggieMike88

    AggieMike88 Touchdown! Greaser! PoA Supporter

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    I can't speak to what to do for the turbo 182, but Ron levy had me configuring for "approach cruise level" resulting in a 100mph IAS at a 5 miles from the IAF. Thus giving me time and slow enough so there is time to correct anything that isn't right.
     
  4. dtuuri

    dtuuri En-Route

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    You can't. One or the other has to give way. Keeping the speed up until the FAF isn't too smart for a non-precision approach, since the MAP usually depends on timing and slowing down after the FAF will screw that up.

    dtuuri
     
  5. brywd

    brywd Pre-takeoff checklist

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    I have no turbo experience, but I'm curious to learn about this. Do you have an engine monitor? I'm curious if you were to pull manifold pressure back 5 inches or so in a scenario like you describe how much of a temperature change actually occurs, and how rapidly. You could try it once and download the data to check. From what I've read shock cooling is a myth, but when you are paying for the maintenance it pays to be overly cautious. And again, I have never flown with a turbo so I put this out there hoping others debunk it. Best way to learn on a forum I find... Say something that is incorrect and the truth will follow promptly. :D
     
  6. Jay Honeck

    Jay Honeck Touchdown! Greaser!

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    What happens to your engine at shut down? In Wisconsin, the cylinder heads that were 350 degrees were suddenly immersed in air that could be 365 degrees colder.

    And the exhaust system was even worse, going from cherry red to -15, instantly.

    My point is this: Gradual power changes are always best, for any machine's long life. However, I think we, as owners, get a little too wound up about shock cooling.
     
  7. Dan Thomas

    Dan Thomas En-Route

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    And all that happens with still air in the cowling, not 100-knot air. Stick you arm into that cowling right after shutdown and see if it's ambient temperature in there.

    Exhaust systems aren't made of cast aluminum.

    That said, there are plenty of people that say shock cooling is a myth. There are jump and tow plane pilots that close the throttle completely and spiral down several thousand feet to the airport, and do that all day, all summer. I remember doing that.

    Dan
     
  8. Clark1961

    Clark1961 Touchdown! Greaser!

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    Have you discussed your concerns with your II? Tuuri is correct, you need to be on speed at the FAF on most non-precision approaches to get the timing right. On a precision approach ya gotta be on speed at the FAF for a stabilized approach (okay, that one gets abused a lot for little airplanes going into busy airports). Hell, I've been asked to keep my speed up on non-precision approaches at busy airports...engine management sorta takes a back seat at that point.

    My A&P is an old airline & freight guy - he swears shock cooling is real and has changed enough cracked cylinders to have a reason for the opinion (or so he says). I haven't cracked a cylinder yet and I try to stage engine power down. One thing I don't do is just pull all the power at once - if ya gotta pull a bunch of power out, leave it at 15" or so, not idle.
     
  9. Clark1961

    Clark1961 Touchdown! Greaser!

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    I appreciate your point Dan and can't argue with experience. I will point out that the engine was probably at 65% power or less when you pulled the throttle. On a normalized or boosted engine, it could be at 75% or more for cruise or outside the IAF.
     
  10. Henning

    Henning Ejection Handle Pulled

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    You can't really do both, so the far more critical issue is the correct approach procedure. In the 310 I can set the power to the bottom of the green arc and then adjust my speed slowly down early out by rolling in trim, first flaps, gear, more flaps, final flaps, then pull throttles back. This gives the engines a nice come down from whatever temp and still allows me to be at target speeds and a 600fpm descent with minimal throttle manipulation.

    2" every 2 minutes is unnecessary, you can draw straight from full power to bottom of the green and not run into what little cooling issue there is. The worst thing is when people shut off the power and shove in full rich for descent and then have the prop drive the engine. That may flow enough fuel that you can have a cooling issue.
     
    Last edited: Jun 29, 2014
  11. nosehair

    nosehair Cleared for Takeoff

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    Ditto
     
  12. Palmpilot

    Palmpilot Touchdown! Greaser! PoA Supporter

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    Re-posted from a red board thread of a few years ago:

    There is an article on line about the experience at a UK gliding club. In addition to only buying new engines and new cylinders, they found that controlling the rate of cooling in the first minute after glider release was very important. After that the temperature was below 200 deg. C (392 deg F), and power was set as needed. They went from 28 cracked cylinders in four years to having it be a rare event.

    http://www.peter2000.co.uk/aviation/misc/shock-cooling.gif
     
  13. Clip4

    Clip4 Pattern Altitude

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    Reduce both MP and RPM to lower end of green arcs.
     
    Last edited: Jun 29, 2014
  14. JHW

    JHW En-Route

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    to avoid shock cooling, don't fly into a lake. If you keep cooling the engine with air, pull off as much power as you want.
     
  15. RotorAndWing

    RotorAndWing Final Approach

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    And the heat will dissipate at a given rate. After shut down take a infrared thermometer and watch the cooling rate of the engine. It will be a gradual cool down, not rapid and sudden.

    Uh, no. It's not "instantly". The metal will cool at a gradual rate in ambient air.


    Agreed.
     
  16. Cpt_Kirk

    Cpt_Kirk En-Route

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    This.
     
  17. dmspilot

    dmspilot Pattern Altitude

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    Why does your CFII tell you that you need to keep your speed up? How fast is "up"?
     
  18. Fearless Tower

    Fearless Tower Touchdown! Greaser!

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    Maybe he is trying to slow to 90 kts 20 miles out? Dunno.
     
  19. poadeleted20

    poadeleted20 Deleted

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    Good advice -- turbochargers are very sensitive to rapid cooling.

    Who told you that, and what justification did they give for saying that? Certainly nothing in the FAR's, AIM, or Instrument Flying/Procedures Handbooks.

    You don't. You either tell your instructor you're not going to trash your engine to make him/her happy, or find another instructor who understands the balance of priorities in flying turbocharged aircraft especially during instrument training.

    It seems to me you're smarter than your instructor. Getting the flaps and gear out early helps slow you down while not having to chop power so much/so fast. It also allows you to follow FAA guidance for making stabilized instrument approaches.

    About six months ago I did some remedial training with a relatively low time pilot flying a C-421 (the FAA Inspectors involved said they consider 400 hours to be kind of low time for an airplane like that) who'd managed to attract the FAA's attention by landing way fast and halfway down the runway out of an instrument approach straight-in to the runway, and then blowing both mains trying to avoid going off the end at a tower-controlled airport. He succeeded in staying on the runway, but taxiing clear was a problem with wheels that were no longer round, and that led the Tower to report the incident to the FSDO. Note that the big-bore turbocharged Continental engines on the 421 are notoriously sensitive to rapid power reductions at high speed.

    I was called upon to prep him for his 709 reexamination ride, and discovered he was trying to fly approaches upwards of 140 knots IAS in an airplane with recommended final approach speeds in the 115-knot range. By getting him to slow down much earlier in the arrival sequence, he was able to fly stabilized approaches at appropriate speeds. We also started using partial flaps and gear extension to add drag early on so he could slow down incrementally without yanking the throttles back so fast. Using these techniques, he was able to make stabilized approaches to the normal touchdown zone and do midfield turnoffs on 7000 foot runways without smoking the tires/brakes. With those changes to his procedures, he passed the 709 ride with ease, and the FAA Inspectors giving the ride (yeah, two of them -- one checking the pilot, the other checking the checker) were very complimentary on how he managed speed, power, and configuration.
     
    Last edited: Jun 29, 2014
  20. JasonM

    JasonM Pattern Altitude

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    Sorry for being a little late to respond and thanks for all your input fellas! ;)


    I do. I will do a little testing and see how far it cools off. I'm not too concerned when doing pattern work, but after a longer flight then all of a sudden going from cruise to 90 ktas approach. Thats a MP jump from 26" to 11".

    That sounds good, but I am being told I have to keep the speed up until reaching the FAF. 15" will be above 90 Ktas and wont allow a decent.

    Thanks, I'll have to check that out.

    ?? how, when, where, why? :)


    I try to stay out of them.


    typically 139 KTAS in cruise. then having to get to 90 with very limited space.


    My CFII. whom has approx 10,000 hours flying GA and been an instructor for a long time. I definitely am not trying to prove him wrong, but 99.999% of his students are flying 172's and going from 100 ktas to 90 ktas, which is a little easier than what I am trying to do.

    Ron, am I grasping from what you are saying, that I should be able to reduce speed slowly prior to entering the approach. I could within all FAR's, AIM, etc. proc.'s, be perfectly legal and acceptable to slow down to my 90 Kt approach speed prior to the IAF? This has been made a big deal of by my CFII and I am only trying to do as instructed.

    How do you teach your students? how far out are they normally setting up for approach speeds?
     
  21. warthog1984

    warthog1984 Cleared for Takeoff

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    :confused: Are you talking about slowing to 90 at the FAF or IAF?

    If you start power reduction before the IAF, you should have enough room to be stabilized and slowed as you cross the FAF.
     
  22. dmspilot

    dmspilot Pattern Altitude

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    Ouch. Listen to Ron. Either persuade your instructor that this is ridiculous, or find a new CFII.

    I slow to the approach speed at or before the IAF unless there is a good reason otherwise (ATC, or a very long initial segment).
     
    Last edited: Jun 29, 2014
  23. James331

    James331 Touchdown! Greaser!

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    Also adjust your mixture to keep target EGTs, 9/10 times your CHTs will follow suite.

    I always go landing config 3nm from the FAF or 1 dot below G/S.

    Don't forget the cowl flaps too!

    As for speeds we have a few speeds, our cruse speed, our terminal area speed, and our speed when we're being vectored for an approach and our inbound speed once established, then we slow to cross the threshold at vref.
     
    Last edited: Jun 29, 2014
  24. FastEddieB

    FastEddieB Final Approach

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    My thought as well.

    At the FAF you should already be at your approach speed and configuration, needing only a power reduction and pitch change to begin descent.

    IIRC, the Cirrus Standardized procedure is to slow to approach speed within 2 minutes of the IAF, to give plenty of time to get slowed down and trimmed.

    I'll see if I can find a reference.
     
  25. Fearless Tower

    Fearless Tower Touchdown! Greaser!

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    So is he wanting you to maintain nearly 140kts until the FAF and then slow to 90?
     
  26. gismo

    gismo Touchdown! Greaser!

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    FWIW, the concept of "shock cooling" is pretty much an OWT. In extreme cases the risk isn't cracked heads but rather scuffing (piston skirts contacting the cylinder walls) due to cylinders cooling much faster than the pistons. A conservative cylinder head cooling rate is 60°F/minute and I'd be surprised if you came close to that going abruptly from cruise power to the bottom of the green in level flight. If your engine monitor records CHTs you ought to dump the data and see what kind of cooling rate you've been seeing, chances are the most significant rate occurs on final and rollout. In some cases the cooling rate that occurs after shutdown when the airplane sits on the ramp pointed into a strong wind exceeds anything normally found in flight.

    The only way I've ever been able to exceed 60°/minute is by reducing power to almost idle and pushing the nose over to descend at a speed higher than cruise. In any case the oft advised one inch of MP per minute is way overkill and IMO detrimental to safe and efficient operation.
     
  27. dell30rb

    dell30rb Final Approach

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    Speed can be dependent on your workload, how rough the air is and how much distance you need to cover.

    Thanks to Garmin there is a 'time to next waypoint' and I try to time things so that I have the gear down and my approach speed of 100 knots about a minute before the FAF.

    Your speed before that is up to you.
     
  28. JasonM

    JasonM Pattern Altitude

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    FAF

    Slowing prior to the IAF would be great. I'm going to stir the pot.

    That's pretty much how I manage the engine now aside from decent when I leave it leaned. I thought opening the cowl flaps was another way of promoting shock cooling?

    That's what I want to do

    Yes sir..
     
  29. poadeleted20

    poadeleted20 Deleted

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    It sure is easier, and it would explain his thinking.

    Absolutely. In fact, most DPE's compliment my trainees on doing just that.

    I establish a series of "gates" or "windows" based on distance out from landing, at we work out a target speed and configuration to be in at each get. I usually try to keep the final segment inside the white arc so you can go full flaps for landing once you break out. I also try to fly the entire approach with approach flaps (10 degrees for a 182) so the initial big pitch trim change that comes with flap extension is done before you start and you don't have to fight it while trying to fly the approach.

    For that Turbo 182, it would probably be something like 90 KIAS on final to give you a good margin under the full flap limit speed of 100 KIAS. How soon to slow to 90 would be based on the trainee's experience -- I'd slow down sooner with a less experienced trainee. So, for someone in initial IR training, I think I'd aim for flying the whole approach at 90 with flaps 10. With more experience, I might say fly much of the approach at 100-120 and slowing down to 90 approaching the FAF, but I consider that a slightly more advanced technique which I'd reserve until you feel comfortable flying approaches at 90 KIAS.

    I'd start slowing from cruise speed probably 5 miles/3 minutes from the IAF, by reducing throttle a couple of inches, configuring the prop full forward, and lowering the flaps to 10. Since the 182 allows flaps 10 at up to 140 KIAS, and you aren't likely to be cruising faster than that, you can stick that drag out with only a minimal throttle reduction. If you need to reduce power further to get down to 90, do an initial reduction to 110 or so at 10 miles and then 90 at 5 miles from the IAF in order to keep your desired 2"/2 minutes MP reduction rate.

    Later on, you can try slowing to 100-110 initially, and then 90 as you approach the FAF. However, during initial IR training, most folks seem to do better if they don't have to reconfigure the plane in the middle of the approach.
     
    Last edited: Jun 30, 2014
  30. poadeleted20

    poadeleted20 Deleted

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    Lance's comments on cylinder head cooling are accurate, but this is a Turbo 182, and turbochargers are far more vulnerable to rapid cooling problems. So, you might not hurt the cylinders, but you could trash the turbo. A TIT gauge is very useful in managing power reductions with turbochargers.
     
  31. JasonM

    JasonM Pattern Altitude

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    Thanks Ron.
     
  32. poadeleted20

    poadeleted20 Deleted

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    Happy to help. I'd be even happier if you called me to hop to WV and finish your training with you -- you've already got Lesson 1 covered. :wink2:
     
  33. RussR

    RussR Line Up and Wait

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    This whole procedure from the OP's CFII baffles me. Slow down AT the FAF? Even for a 100kt to 90kt Cessna 172 that's not correct. Maybe there's a misunderstanding going on between the OP and the CFII?

    The whole purpose of the intermediate segment leading up to the FAF is to give you time to get aligned and ON SPEED, configured as necessary, etc., so that once you hit the FAF you start descending and don't have to worry about that stuff anymore. So you can just pay attention to course and altitude.

    The TERPS even limits the descent gradient in the intermediate segment so that it's relatively flat to allow for this speed reduction and configuration (although the max descent gradient in the intermediate CAN be as steep as that on final - and you'll see that in the mountains - the recommended rate is much lower). Many procedures have a perfectly level intermediate segment for this reason.

    You need to be slowed down before reaching the FAF for a stabilized approach and for timing to have any chance of being close. Establishing approach speed way out there prior to the IAF is the ultra-conservative version of this, but works just fine.

    Even in a 172 it doesn't make sense to reach the FAF at 100, THEN slow to 90 while starting your descent.
     
  34. AggieMike88

    AggieMike88 Touchdown! Greaser! PoA Supporter

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    As one of his recent students, I can answer this. He taught me to be configured for approach level (1975 C182P, 100MPH, 16" MP, 2350RPM indicated, 10° Flaps) at least 3 minutes or 5 miles before entering the initial approach environment.

    This gave me plenty of time to get stabilized, run checklists, and re-brief the approach before hitting the IAF and starting my run.


    For those interested, attached is the configuration chart Ron and I worked up early in our IFR lessons. I use this every time I fly and like checklists, makes arrival and approaches much simpler and stabilized.
     

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  35. Witmo

    Witmo Pattern Altitude

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    But what do you use if you're not inverted?
     
  36. FastEddieB

    FastEddieB Final Approach

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    Found this, from a PowerPoint presentation for the SR22:

    [​IMG]
     
  37. JasonM

    JasonM Pattern Altitude

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    I've been doing this now for the past 14 hours of my IFR training, well. whever amount of that is the actual approach. I think the approaches we did prior had longer sections from the FAF to the runway or MAP and it never seemed like a big problem. But the last few approaches had shorter gaps and I struggled to get down/slow down fast enough.

    Not sure how much that would cost, but I am sitting with about 22 hours of hood time and my knowledge test is complete.

    After reading these responses, one would think there was some mis-communication.

    Thanks, That is very close to my numbers we established as well.

    I thought the SR22 was faster than that. It can only do a 100 on final? ;)

    In all seriousness, thanks. That is how I would like to approach the approach.
     
  38. poadeleted20

    poadeleted20 Deleted

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    Good. Now go explain that to your instructor. :eek:
     
  39. gismo

    gismo Touchdown! Greaser!

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    AFaIK the only things that will trash a turbo are insufficient oil flow (kills them quickly if really low otherwise slowly) or exceeding TIT limits (the blades expand enough to rub the housing). During normal operation the exhaust flow through the turbine will vary hundreds of degrees over as little as a few seconds and this shouldn't harm the turbo at all. There has been some concern about cylinder temp cooling rates specific to turbo'd engines but only because the POH recommendations often generate excessive CHTs which are the real cause of many cylinder problems rather than the cooling rates.
     
    Last edited: Jun 30, 2014
  40. skidoo

    skidoo Line Up and Wait

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    Lots of interesting info and viewpoints here... I have the same T182T as you and I found that I had to become my own expert on this one. Seems no instructor in the area knows it well. So, you may have to learn as much as you can and become your own expert, taking into account all relevant information.

    In IR training, I found that the most frustrating part was managing the power, prop, mixture while doing all the other IR procedures. Once I get it set and tweaked for cruise, instructor says climb, turn to heading xxx, etc... I think this training was the worst affects on engine management. But, it survived. At least now, I can anticipate a bit more ahead of time, and there isn't the need to be making all those power management changes in a short period of time.

    I chose to do my cruise power settings at 20", 2100 rpm during training (except long x-country). That way, there was less extreme changes, saved on fuel, and there was more time to think.

    After cruise, I typically will need to descend at least 10 minutes out. A few minutes before that, I can open the cowl flaps to begin some cooling. Then, when it is time to descend, close the cowl flaps, and pull back power to maintain speed as I descend. When time to level off and slow down, open the cowl flaps either full or partial, and set power to 20" for about 105 to 110 IAS. From there just adjust the MP as needed for slower.

    I find that Once I set the mixture at a particular MP, I can reduce MP without touching mixture, as long as I don't increase MP above that which I adjusted it for. This helps save me some workload. For example, If I set MP for 20" or 24" and lean for 1585 TIT, then I can reduce MP or increase it back to 20" or 24", but not above. If I need to increase MP to above that set level, then I will need to re-adjust mixture. If I am cruising at 24" and get to a point where 20" is all I will need for the remainder, I would re-lean for 20".

    I have not been trained to do it this way and some of this may not be optimal, but it seems to work for me. It is just what I figured on what to do based on all information I had collected and from experimenting.