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10 Answers

aproach speed

Asked by: 11746 views Aerodynamics

is the aproach speed of an aircraft same for airfields of difeerent field elevation.  suppose aproach speed of an aircraft at MSL is 80 knots. should the same aproach speed be used for landing at an airfield having field elevation above 5000 feet?

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10 Answers

  1. Wes Beard on May 20, 2012

    No.  Typically the approach speed is established at 1.3VS0 or 1.3 times the stalling speed in the landing configuration.  The stalling speed will increase with a increase in density altitude.
    However, in small traning aircraft the difference may only be a couple knots or so and not big enough to worry about.  In transport category jets, the speeds could differ by 10-15knots.

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  2. Nathan Parker on May 20, 2012

    Indicated airspeeds, which is what you use for all aircraft operations, are independent of density altitude.  This includes the stalling speed of the aircraft.  The magic which makes this work is that the aircraft’s true airspeed increases at high density altitudes just enough so that the indicated airspeed remains a constant.  (Many people draw the wrong conclusion from the Lift formula because they don’t understand that “V” in the formula is true airspeed.)
    However, Mach number does have an impact on the stall speed, but Mach number is so small on approach that I doubt this has any significant impact on any category of aircraft.

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  3. Wes Beard on May 20, 2012

    Mach number has no bearing on this conversation.  I would agree with you that we are talking about indicated airspeed and true airspeed is what is in the lift formula.  However, read my post again.  I said the airspeed (meaning indicated airspeed) only changes a couple of knots. 
    Have you flown a jet before?  If you had, you would realize that the indicated airspeed increases with an increase in density altitude

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  4. Nathan Parker on May 20, 2012

    “Have you flown a jet before?  If you had, you would realize that the indicated airspeed increases with an increase in density altitude”
    Physics doesn’t change with the propulsion system.  If you agree we’re talking about the lift formula, then there is no justification for the IAS to change at all, since there is no change in IAS stall speed.  This can be demonstrated with simple algebra.
    If you have published data showing different approach speeds with DA being the only variable, I’d be interested in seeing it.  I can imagine reasons other than aerodynamics as to why this could be true.

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  5. Kris Kortokrax on May 23, 2012

    Your attempt to discredit Nathan’s response by asking if he had ever flown a jet is not valid.
    Commercial applicants are asked to describe pressurization although they may not have flown pressurized aircraft.  Instructors may be able to explain turbocharging or supercharging without having flown those aircraft.
    I have not flown a jet, however, I am perfectly capable of looking at performance charts and understanding the information presented.
    I had the opportunity to look at the tab data in a Lear 35 POH and a Citation POH.  In both cases, the approach speed was predicated upon landing weight and nothing else.
    Landing distance is affected by altitude, temperature, brake energy etc, but the approach speed is not.

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  6. Wes Beard on May 23, 2012

    You’re right.  I was rude and I apologize.  I have looked on the internet for the LR-45 and CL-604 performance charts and can’t find them.  However, I have them in book form and the LR-45 gains one knot for VREF as it climbs in altitude.  The CL-604 increases its VREF by 11 knots from sea level to 14,000FT.  It is quite possible that manufacturers will use a more conservative number than to calculate the difference density altitude makes on the landing numbers.  I don’t know.  It is an interesting question.  I do know the CL-604 landing numbers are based on 1.2VSR (referenced stall speed due to a stick pusher) instead of 1.3VSO and can explain the increase in the numbers.

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  7. Kris Kortokrax on May 24, 2012

    I have a friend who has flown a Lear 45 and has the books.  Yes, the Vref at 10000 feet is one knot higher than Vref at sea level.  I don’t believe that altitude or airspeed accounts for the difference.
    21.125(b)(2)(i) states:
    In non-icing conditions, VREFmay not be less than:
    (A) 1.23 VSR0;
    (B) VMCLestablished under §25.149(f); and
    (C) A speed that provides the maneuvering capability specified in §25.143(h).
    This must be the reason for the variance you see in your charts.

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  8. Kris Kortokrax on May 24, 2012

    Correction, I should have said that neither altitude nor TEMPERATURE (not airspeed) accounts for the difference.

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  9. Nathan Parker on May 24, 2012

    I also downloaded a Lear 45 chart and saw the same 1 knot increase in Vref.  However, the stall speed graph also shows a small increase in calibrated stall speed above about 5,000 feet.
    I interpret this as a Reynolds number effect.  Airfoils typically have a different Clmax with different ranges of Reynolds numbers, which is a number that includes factors such as air density, true airspeed, wing chord length, and the viscosity of air.  If you look at lift curves in such books as “Theory of Wing Sections”, you’ll see different curves for different Reynolds numbers and the magnitude of these differences vary among airfoils.  It would seem that the Lear is among the more senstive to this parameter if you’re seeing changes at altitudes as low as 5,000 feet.

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  10. JB on Aug 13, 2012

    Guys, this is a case of “measure it with a micrometer, mark it with a crayon, and cut it with an axe”. Unless you fly into Quito, Mexico City, or La Paz, it’s basically the same. Operationally, what you really need be aware of is that your TRUE airspeed and hence groundspeed will be higher (increased stopping distance). The air is thinner at the higher elevation and that needs to be taken into account during the flare as well.

    I wish I could put a JPEG on here to show you but, take the Triple Seven for example:
    At 575,000 pounds landing weight the Vref is 151 at Sea Level all the way through 9,000 feet Pressure Altitude. It then jumps one knot. That’s it. I haven’t met a pilot who can hold their approach speed to within one knot all the way down final.

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