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4 Answers

How to select a cruise altitude?

Asked by: 6170 views ,
Aerodynamics, Airspace, Commercial Pilot, General Aviation, Instrument Rating, Private Pilot, Student Pilot

I fully understand how to select an appropriate altitude based on eastbound vs westbound, VFR vs IFR, and obstacle clearance, but what about beyond that? Say I am planning an easbound VFR flight. How should I choose between 5,500, 7,500, 9,500, etc.? I'm sure wind reports play a big factor, but what about the operating efficiency of the engine? Or a higher altitude giving you more time and options in the event of an engine failure (or trapping you in the plane for a longer period of time if there is a fire)?

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4 Answers

  1. Best Answer

    John D. Collins on Dec 02, 2011

    This is a very open question to which there is no correct answer.  You have to evaluate all the factors and decide what is more important to you, with a given set of conditions on each flight. For me, I would also consider turbulence, total flight duration, fuel required, is it a day or night flight, what are the weather issues, the temperature, passenger comfort, oxygen, etc. Regarding efficiency, if you are able to maintain a constant power, then higher altitudes will be more effecient at cruise, but getting to altitude will consume more fuel, so it is a tradeoff, whith some block to block distances being more effecient and others less effecient for a particular flight. I give a fair amount of consideration to passenger comfort, so if it is hot, I may climb higher to get a cooler cabin temperatur, but I have to trade that off with the possible need of oxygen and a longer descent with possilbe discomfort for those with sensitive ears. It is interesting to note that the speed for best range is fairly independent of the altitude, in my Bonanza, it is probably close to 130 Kts. In otherwords, at that indicated speed, the no wind range of the aircraft is not affected much by altitude, but because a CAS of 130 Kts at 4500 feet on an ISA temperature day gives a TAS of 139 Kts, whereas a CAS of 130 Kts at 12500 MSL on an ISA day gives a TAS of 157 Kts, so you will go roughly as far, but take less time to accomplish it. On the otherhand if effeciency isn’t the issue and you want to get there the fastest, an altitude of 7500 feet will still give you 75% power and provide the fastest cruise speed for a non stop flight. If you don’t have sufficient fuel to complete the flight at 75%, but can avoid a fuel stop at 60% power, that might be a better choice.  So as one wise sage frequently says, it depends.

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  2. Abdel Shabazz on Dec 02, 2011

    To add to John Collins’ information, terrain and meteorological conditions can also influence your decision as to an appropriate altitude to maintain. But as he mentioned, it truly is a open-ended question, and depends upon your mission at the time.

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  3. Bill Trussell on Dec 08, 2011

    Expanding on one of your suggested considerations, I did experience an engine failure on an eastbound cross country.  Fortunately it occurred in a twin, so at worst it was a controlled decent.  The end result was an engine out landing but under control at an appropriate airport.  ATC was fully engaged in my issue and was very helpful at the time.  I would suggest that if you can afford the climb to a higher altitude and conditions permit it is worth it to go higher.  The same situation on a west bound flight is harder to put a value on without knowing about terrain and upper level winds.  In my case I was at 9000 when the engine gave up due to a oil pump shaft failure.  I had an appropriate amount of time and several options!

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  4. Bob Watson on Dec 10, 2011

    Let’s see, here’s what I consider (more or less in order):
    1) terrain clearance (without that, the rest is academic 🙂 ) But varying the route a bit can alter that value. Some routes over nasty terrain might require a much higher clearance than routes over less formidable terrain.
    2) navigation reception. Not a problem with GPS, but if you’re using VORs, it can make a difference
    3) Wind. this is generally stronger as you go higher so it’s going your way, it might be worth  the climb. But this varies a lot with the conditions.
    Other considerations include:
    a) the view: is it worth staying low to see, or will it be hidden by an undercast and not a concern?
    b) Will I or my pax need O2? Not a big deal but, for me, it means not forgetting to bring the tank (I’m a renter).
    c) freezing and icing forecasts (it’s that time of year, again!)
    d) distance (this factors into #1 in that a short hop probably isn’t going to make it worth the effort to climb any higher than necessary.
    e) the a/c performance is a function of my wind consideration (#3) in that whether or not it’s worth the climb depends on the resulting ground speed at the altitude.

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