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3 Answers

What is favorable altitude for X-C ? (It’s Not about regulation, wx…)

Asked by: 1869 views , , ,
Aerodynamics, Aircraft Systems

Hello.

I'm a commercial pilot and currently training with multi-engine add-on course now.

It's a pretty shame for me that I'm asking these questions at commercial pilot's level.

As you guys know, as altitude increase, there's a thinner air so less drag and it increases fuel efficiency. That's why commercial air-lines usually select high altitude for high TAS.

When I was training with commercial pilot stage, I went a X-C to another airport and selected 10,000ft altitude.

In that time, IAS indicates roughly 80KTS and as far as I remember, TAS was 95KTS I think.

So It takes almost 3 hours to get there.

But When I was taking back from that airport, selected 5,000ft and IAS was 100 and TAS was 110.

Of course, It takes less hours.

 

My question is this.

As altitude increase, we can meet less drag, but as long as we are flying with Natural Aspiration engines, there's a thinner air are exist, so Raising the altitude unconditionally is not a right answer.

Then, Is the lower altitude good for speed? well..... That makes confusing me. cause less altitude may increase air molecules, but increases air drag as well. and TAS also less than the high altitude.

Exclude FAR(Crusing altitude), wind condition, aviation weather, How can I select most favorable altitude for X-C?

Is there any charts or diagram such as like L/D max graph?

 

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3 Answers



  1. Mark Kolber on Jun 21, 2019

    There are two issues here. Airspeed and ground speed.

    Airspeed. Due to the thinner air at altitude, the same IAS means increased TAS. But there\’s a catch. The thinner air at altitude also means less engine power and less airfoil efficiency, so at some point you can\’t get enough IAS to produce that higher TAS. To simplify, in theory, a Cessna 172 flying at 12,000 feet at 120 KIAS will have a TAS of 149 KTS (using the 2%;1000\’ rule of thumb). Problem is, you ain\’t getting 120 KIAS in a 172 at 12,000\’.

    That’s what your POH power tables are for. To help you select and identify the settings which will produce certain TAS at various altitudes and temperatures (density altitudes). Most airplanes have a density altitude sweet spot where speed and efficiency are maximized.

    Groundspeed. But the inquiry doesn\’t stop at TAS. If you could get 149 TAS out of that 172 at 12,000 feet, you might also find a 40 KT headwind, making 4,000\’ with its 10 KT tailwind a far better choice.

    So, the optimal altitude for a cross country based solely on speed is going to involve consideration of both aircraft TAS capability and forecast winds.

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  2. Warren Webb Jr on Jun 26, 2019

    For a C172S the altitudes/maximum true airspeeds are 2000 ft/116.5 tas, 4000 ft/118.5 tas, 6000 ft/120.5 tas, 8000 ft/123.2 tas, 10,000 ft/123.0 tas, 12,000ft/119.0. It’s really a small spread, so the primary factor in choosing an altitude for a normally aspirated airplane will be the winds. Check the forecasts carefully – sometimes the weaker winds are higher.

    If you are lucky enough to be in a SR22T (turbocharged), 2000 ft/170 tas but 25,000 ft/213 tas, a gain of 25% in TAS. Then altitude becomes a significant consideration for cruise.

    Airspace, clouds, and terrain will be factors also, but for normally aspirated airplanes, winds are the primary factor for choosing altitude since there is such a small spread in TAS.

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  3. Paul H. on Sep 13, 2019

    The answer to this one is in your plane’s manual.

    First, find the maximum power percentage for sustained cruise flight. It’s 75% power in many light planes.

    Now, look at the power setting charts for 75% power (or whatever your highest one is), and look at the power settings and TAS results at various altitudes. The highest altitude listed under the highest power setting is going to be your fastest cruise speed. If you fly any higher than that, your engine won’t have the horsepower to make 75% power (or whatever) anymore. You’ll have to use a setting for 65% or something.

    Also, your manual probably lists “best cruise” speed on another page along with a bunch of other important numbers. That “best cruise” speed probably has an altitude listed with it. This will be the same as the number you found in your power settings.

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