Speed vs high alt. aerodroms PMDG 737

[gura75 0]

Hi This question concerns high altitude airports, thinner density and indicated speed vs. true speed vs PMDG 737 planes. ".....density altitude affects the landing performance of an airplane High temperature and high elevation will cause an increase in the landing roll because the true airspeed is higher than the indicated airspeed. Therefore, even though using the same indicated airspeed for approach and landing that is appropriate for sea level operations, the true airspeed is faster, resulting in a faster groundspeed (with a given wind condition). The increase in groundspeed naturally makes the landing distance longer and should be carefully considered when landing at a high elevation field, particularly if the field is short........" My question is as follows: Is this crucial fact integrated in pmdg 737 package? Will indicated air speed [iAS] really be slower than true air speed [TAS] when choosing high altitude aerodome in FSX? Sincerely

[ZachLW 37]

Yes Geir. That is a standard feature of FSX.When landing at Mexico City, the ground really wizzes by!

Edited January 9, 2012 by ZachLW

[gura75 0]

Thakns for a quick response. So.....when FMC calculates landing speeds vs flap settings its all taken care off vs. the destination airport's altitude? Or do pilots have to adjust landing speeds according to the destantion airport's altitude? Let me give you an example. Imagine your FMC calculates 125 knots or (mph) flaps 40 setting at RWY airport MHTG ( at 3297 feet) adjusted for wind correction. Should one stick to these settings or should one subtract some speed?

[ZachLW 37]

Thakns for a quick response. So.....when FMC calculates landing speeds vs flap settings its all taken care off vs. the destination airport's altitude? Or do pilots have to adjust landing speeds according to the destantion airport's altitude? Let me give you an example. Imagine your FMC calculates 125 knots or (mph) flaps 40 setting at RWY airport MHTG ( at 3297 feet) adjusted for wind correction. Should one stick to these settings or should one subtract some speed?

The FMC is giving you indicated airspeed. So vref doesn't change regardless of field elevation (and wind since you mentioned that). The FMC also doesn't correct approach speeds for "wind correction". Vref is calculated by weight and flaps setting. That's it.So basically:

when FMC calculates landing speeds vs flap settings its all taken care of

Someone else can take it from here. I need sleep! :( Edited January 9, 2012 by ZachLW

[Ponk 51]

Always put the settings it suggests in! Dont make any manual calculations. Runways are generally longer when they have a high elevation to compensate for the thinner air. The pitot head feels thinner air pushing against it so it gives a false reading on the low side ie ias is less the tas or gs but the wing also has to deal with the thin air so you need to have the faster gs to stay in the air.

[ZachLW 37]

Always put the settings it suggests in! Dont make any manual calculations. Runways are generally longer when they have a high elevation to compensate for the thinner air. The pitot head feels thinner air pushing against it so it gives a false reading on the low side ie ias is less the tas or gs but the wing also has to deal with the thin air so you need to have the faster gs to stay in the air.

Not exactly.Please remember there's a reason stall speed is based on indicated airspeed. With that in mind; Vref is closely related to... you guessed it!: Stall speed.

The pitot head feels thinner air pushing against it so it gives a false reading

The reading isn't "false" (we could get into calibration errors but I'm not touching that at 1:30AM).

on the low side ie ias is less the tas or gs but the wing also has to deal with the thin air so you need to have the faster gs to stay in the air.

I'm trying to make sense of that one. Forget GS, it's not important unless we're worried about skidding off of the end of the runway or bursting our tires. Not a factor for Vref. EDIT: I see what you're trying to say. Edited January 9, 2012 by ZachLW

[japascoe 98]

The indicated airspeed is really a measure of the dynamic pressure of the air on your wing, which depends on both the temperature and the airspeed. Indicated airspeed is the speed relative to the air, if the air had a density of 1.225 kg/m^3, which is the density of air at sea level with an atmospheric temperature of +15 C. If the density is lower than that number, your actual speed relative to the air (the true airspeed) will be higher than the airspeed that is indicated.For an airplane to fly, what matters is the dynamic pressure of the air on your wing, in other words the indicated airspeed. If the density of the air goes down you need to have a faster speed relative to the air (so a faster true airspeed) in order to have sufficient lift, so the true airspeed at which you stall* goes up. However, although your true airspeed will be faster, because the density is lower your indicated airspeed will be the same.As Mr Waddell indicated Vref is related to the stall speed, to be precise it is 1.3 times the stall speed. There's nothing magic about the 1.3, it was chosen to provide a sufficient safety margin above the stall speed in case of sudden wind shifts or pilot error. Thus the numbers which the FMC produces, which are indicated airspeeds, are always correct and should always be used.How much runway you actually need in order to stop the plane depends on your speed relative to the ground (i.e. your groundspeed), which depends on your true airspeed and on the wind speed and direction. Groundspeed is only used for navigation though, and not for the actual flying of the plane.*Note for the pedantic / advanced readers: Actually stalling has nothing to do with speed, you stall when you exceed the critical angle of attack of the wing, regardless of your speed. Stall speed is the slowest speed at which you can produce an amount of lift equal to the weight of your plane without exceeding the critical angle of attack.