Critical Altitude and Density Altitude (with Turbo)

Discussion in 'Hangar Talk' started by BGF_Yankee, Feb 5, 2019.

  1. BGF_Yankee

    BGF_Yankee Line Up and Wait

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    I am sure this is supposed to be a relatively simple topic to understand, but I admit that I do not know this answer. Is the critical altitude of an aircraft equipped with turbocharged engines affected by density altitude? i.e., if density altitude is high at the departure airport, will the critical altitude be affected as well for that flight, or is critical altitude an absolute regardless of density altitude?
     
  2. flyingcheesehead

    flyingcheesehead Touchdown! Greaser!

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    Critical altitude is an absolute in terms of some kind of altitude... I believe it is density altitude, but not 100% sure of that. Basically, critical altitude would be where the turbo begins not being able to generate full rated manifold pressure. On a fixed-wastegate system like the Seneca, that's the altitude at which you reach full throttle. On variable-wastegate systems, however they're controlled, that would be where the wastegate is fully closed.
     
  3. BGF_Yankee

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    Not being snippy, but I know what the definition of Critical Altitude and what it is supposed to be for my aircraft. Thank you, and I didn't mean to portray that I did not. For my Seneca though (to be a little more precise), it's the altitude where you can no longer maintain 40" of Manifold Pressure. Yes, throttle will be wrapped out, but that's what my service manual and POH say I believe.

    My question is more does Density Altitude affect it? In a hypothetical situation, I take off in my plane at my home base, KBGF. The elevation here (for all intents and purposes) is 1,000'. The density altitude today was 2,100'. Does this extend all the way up? Critical Altitude in my POH says it should be between 11,500' and 12,500'. Do I lose 1,100' from the critical altitude? Maybe this is more a question of meteorology and how an increase in altitude affects Density Altitude.

    Can I reach 40" of MP on the ground here? Yes, easily. But does the plane's engine think that it's at 1,000' here or at 2,100' in this scenario? How does that translate through the climb? Will my Indicated Altitude coincide with the published Critical Altitude, or will this fluctuate?
     
  4. Heftiger

    Heftiger Pre-takeoff checklist

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    I would say it’s density altitude. The engine is always operating under density altitude conditions. The real question then is at 12,000 ft MSL, what’s the density altitude on that given day? I must admit, I’ve never done this calculation in flight.
     
  5. hindsight2020

    hindsight2020 En-Route

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    Yes, it is my understanding that the critical altitude will be affected by density. Though the density altitude is affected by humidity as well as temperature, temperature is the driving factor. Your power tables have a factor to add or subtract in order to compensate for lower or higher temperature from standard atmosphere. In the case of a hot day, you will run out of throttle at a lower physical elevation, by virtue of the much higher density altitude, because you require a higher indicated manifold pressure in order to compensate for higher temperature, which correlates to a higher density altitude.

    Look at the differences in performance of your engine setup when an intercooler is installed. Temperature is lower while providing the same pressure, thence, manifold pressure required is lower in order to make the same target power. As such, the ability to develop rated power at higher altitudes effectively increases the critical altitude of your installation. The density altitude your engine is seeing is lower, by virtue of this temperature reduction provided by the intercooler. This is why the power table MPs are modified down when an intercooler is installed. This behavior seems to corroborate the idea critical altitude is indeed a density altitude.
     
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  6. BGF_Yankee

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    Great explanation, thank you.
     
  7. flyingcheesehead

    flyingcheesehead Touchdown! Greaser!

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    Good. Sorry, didn't mean to provide a basic answer but hopefully it'll help the next turbo rookie to stumble across this thread. ;)
     
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  8. midlifeflyer

    midlifeflyer Final Approach

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    Good answer by @hindsight2020.

    But just checking since this is the way you posed your question: Why would the critical altitude be affected by the density altitude at the departure airport (unless, of course, the density altitude on the ground at the airport happens to be above the critical altitude :eek:)?
     
  9. Checkout_my_Six

    Checkout_my_Six Final Approach

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    here I thought critical altitude had to do with some minimum climb capability (power output)....like say 100 fpm?

    which for a turbo charged engine is well above the closed waste gate point....it's the point at which the power output is below a minimum.
     
  10. midlifeflyer

    midlifeflyer Final Approach

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    "Critical altitude" is a turbo term. It's the altitude above which the turbo can't maintain sea level MP.

    Full definition is in FAR 1.1
     
  11. flyingcheesehead

    flyingcheesehead Touchdown! Greaser!

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    No, what you're describing is the service ceiling, which is the point at which the airplane can only climb 100 fpm. This is true of all airplanes - Normally aspirated ones just have a lower service ceiling.

    Single-engine service ceiling is something you'll find on multi-engine aircraft, and that is the altitude at which the plane can only climb 50 fpm with an engine out.

    Critical altitude is where a turbo is doing everything it can to maintain full rated manifold pressure. The plane can fly well above the critical altitude, but manifold pressure will decrease as it climbs.
     
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  12. Checkout_my_Six

    Checkout_my_Six Final Approach

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    yup....I was too lazy to look it up. ;)
     
  13. midlifeflyer

    midlifeflyer Final Approach

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    Me too. But since I flew and taught in turbos, I had a hint :)
     
  14. Checkout_my_Six

    Checkout_my_Six Final Approach

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    Well....flying one doesn't mean anything. And I speak from experience. o_O
     
  15. midlifeflyer

    midlifeflyer Final Approach

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    Yeah but I had to know (just) enough to teach transitions to them. That "instructional knowledge thing," you know :happydance:
     
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  16. BGF_Yankee

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    I was just curious if the difference between Density Altitude and Airport Elevation/Indicated Altitude held all the way up as you climb. i.e. in my question, there's an 1,100' difference. I've never thought about it before and was having a time Googling it without getting basic explanations of airports with high density altitude. Based on my own playing with the GTN650 in flight the other day though, it doesn't seem like that's the way it works exactly. While my Density Altitude on the ground was 1,100' above the airport elevation, according to the GTN650, Density Altitude at 12,000' was actually 125' below Indicated Altitude. Further, was just curious if Critical Altitude was an expression of only Indicated Altitude or Density Altitude. Density does make more sense after the explanation from Hindsight. I have been trying to get my engines rigged properly, and after making many test flights up through 12,000', the question struck me of how Density Altitude would affect the numbers that I was seeing.

    I WILL say though that after spending a year managing the Leadville-Lake County Airport (KLXV) that there were several times Density Altitude was quite high in the summer. I'm supposed to be able to maintain 40" all the way up to between 11,500' and 12,500'. KLXV is surveyed at 9,934', but with a warmer day in the summer, I probably wouldn't be able to hit 40" on the ground.

    10 12000 75 Percent .jpg
     
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  17. flyingcheesehead

    flyingcheesehead Touchdown! Greaser!

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    The relationship would only remain the same if the temperature profile in the atmosphere exactly matched the standard lapse rate, which of course happens approximately never.

    Cool. The first time I visited KLXV was in August of 2008, and the DA at takeoff was 12,200 feet!
     
  18. midlifeflyer

    midlifeflyer Final Approach

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    Basically, if you are talking about performance. Engine power, airfoil performance, depend on density altitude, not pressure or absolute altitude.
     
  19. Heftiger

    Heftiger Pre-takeoff checklist

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    Caution thread drift:

    My TIO540 has the same TIT limit that you have. The other day I was flying at night with the TIT around 1525 (EGTs also high 1400 low 1500) and I could see the glow under the cowling. Made me a little uncomfortable so I richened it up to 1450 TIT and the glow went away. Any concerns running at those temps you see on the picture?
     
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  20. BGF_Yankee

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    I just took those real quick doing a check flight. I usually try to keep TIT around 1450°-1500°. I leveled off, set up how you see, took the photo, and then started a descent. According to the POH, that's actually about 0.5 GPH more than the power chart says. I like to err on the richer side though. My CHTs are typically a bit cooler in cruise when I'm going anywhere. Was just opening it up a minute.