Speed of A320neo

Hi everyone,

 

I have been flying the Airbus A320neo a lot lately.

 

I have noticed that the allowable speed seems to vary depending upon the altitude. However, the maximum speed that I have seen is about 350 knots. According to Wikipedia, in any case, the speed should be allowed to go up to fully 450 knots, maybe a little more.

 

Why is there such a huge discrepancy, please?

 

Thank you.

 

Stanley

Probably just another thing in this sim that doesn't work right. Asobo's quality control is nonexistent.

True air speed should be over 450 knots, but that isn't what your instruments display. FAA has speed limits for various altitudes. For example, you can't exceed 250 knots below 10,000 feet. As the air gets thinner, the displayed speed is less, but the true air speed is still the same. Again, your instruments don't read true air speed!

Hi plainsman,

 

OK. So at 9,500 feet, true airspeed = indicated airspeed = 250 knots, for example. Is that correct?

 

But at 38,000 feet, true airspeed = 450 knots, but indicated airspeed = 250 knots. Is this what you are saying?

 

I am making up numbers. Please make up your own numbers so that I clearly understand your point. This is important.

 

Thank you.

 

Stanley

Indicated and true airspeed are rarely the same, and are used for different things. Indicated is basically the raw pressure reading from the pitot system, while true airspeed is the indicated corrected for temperature and pressure.

 

There are a number of articles and YouTube videos that explain the differences.

 

https://en.wikipedia.org/wiki/Airspeed

 

https://www.aopa.org/news-and-media/all-news/2005/september/flight-training-magazine/truth-in-airspeed

 

Edited November 10, 2020 by loki

On a simple explanation, indicated air speed drops about 2% per thousand feet, but this is a rough ~ in that temperature and atmospheric pressure have to be accounted for (the drop is a function of air density, how many molecules get inside the tube). So at 10,000 feet indicated air speed is about 20% less than true air speed. Now there is another important speed, ground speed. If you fly into a 30 knot headwind, you have a different ground speed than your true air speed.

OK. So at 9,500 feet, true airspeed = indicated airspeed = 250 knots, for example. Is that correct?

Nope. At sea level under standard conditions* indicated airspeed (IAS) and true airspeed )TAS) are the same, but those conditions rarely obtain. IAS is important because it defines some of the limitation of the aircraft and because it is roughly constant in terms of how an aircraft stalls and several other factors.

So at 9500 feet an indicated airspeed of 250 knots, TAS will be around 295-300 kts, depending on temperature and barometer.

 

But IAS comes from ram air pressure into the pitot tube and the calibrations of the instruments are figured under the above-mentioned standard conditions*, but as air density decreases the IAS decreases for the SAME TAS. And air density changes with barometric pressure (including altitude changes) and with temperature and humidity, thus we also have density altitude#, though it's not shown on your instruments.

 

Note that at 18,000 feet the air density is roughly half that at sea level, such that at a given TAS the pitot tube (and gauges) receive much less air PRESSURE because the air is less dense, so the indicator won't read as high.

 

=============================================================

* Standard conditions are 59º F (or 15º C) at sea level at 29.92 Hg" on the barometer, and 0% humidity. Any change from these affects the airspeed indicator and the altimeter.

 

# Density Altitude is the altitude that the airplane "thinks" that it is, due to changes in temperature and barometric pressure, so that on a hot day in Denver (5200 feet) density altitude may be close to 9,000 feet. In other words, the aircraft performs according to density altitude, whether stall, ISA-TAS conversion, takeoff and landing distances, rate of climb, and many other factors.

Edited November 10, 2020 by lnuss

Well with my IAS well below the red warning dashes I was getting an overspeed warning last night. It does seem difficult to coax it above Mach 0.80 which seems rather slow for a modern airliner.

Vern, the A320 has a Maximum operating speed of M 0.82 according to Airbus. It will typically cruise at M 0.76-0.80. A Boeing B737NG has a typical cruise of M 0.76-0.785, and the 737-300 to 737-500 are a little slower than that. The 787 and 747 have higher typical cruising speeds than either.

Hi everyone,

 

Let me just get this issue buttoned down, please.

 

When my A320neo reads a speed of 350 knots, what is the unit? Is it indicated airspeed, calibrated airspeed, true airspeed, or ground speed?

 

Thank you.

 

Stanley

The unit is knots, but it is indicated airspeed.

The unit is knots, but it is indicated airspeed.

 

 

Hi plainsman,

 

OK. So, it is KIAS. Good.

 

Now, when I read in Wikipedia that the maximum speed of the A320neo is about 450 knots, what kind of airspeed is that, please?

 

Stanley

That 450 kts speed is True Airspeed. Note, too, that maximum speed does not mean cruise speed, nor does it mean that the aircraft can achieve that under all conditions. Usually that max speed is achieved only at a specific altitude under specific conditions. At all other times it will be slower. And aircraft are not usually rated to cruise at max speed, but rather at a slower, typical cruise speed.

 

You might also not that, for jet aircraft the max speed and max cruise speed are rated in percent of mach 1, NOT in either indicated or true airspeed (mach IS one form of TAS rating, though).

 

Aviation is far from intuitive, as you've no doubt noticed by now.

Very good. I thank everyone for their great and instructive help.

I made screens of the A320 at 2 positions, one rolling down the runway at Laramie, Wyoming and then at cruise at 37,000 feet at M 0.793. You will note the arrow on the left is IAS. The computer calculates the TAS and GS and those are displayed on the screen to the right under the two smaller arrows.

I posted this by mistake. Just look at the next post, please. Edited November 10, 2020 by Stanley777

Very helpful, plainsman.

 

One last question, please. On the diagrams, I see numbers on the right side in the second row. In the first graphic, it is ---/01, and in the second graphic, it is 270/02.

 

What are those numbers, please?

Edited November 10, 2020 by Stanley777

I think those are wind parameters,. but I am not sure, and I don't want to mislead anyone??

Yes they are wind direction and speed. The arrow below them shows the wind direction in relation to the current heading of the aircraft.

 

Sent from my KFDOWI using Tapatalk

Hi everyone,

 

Thank you all for clarifying this issue of indicated airspeed and true airspeed. I understand everything much better now.

 

I had one more question, please.

 

However one measures it, can an aircraft fly faster at cruise altitude -- let's say 30,000 feet or higher -- than at a lower altitude? If so, why? Does it have anything to do with the density of the air?

 

Thank you.

 

Stanley

can an aircraft fly faster at cruise altitude -- let's say 30,000 feet or higher -- than at a lower altitude?

Not generally. But at a given TAS it will require a lot less power, which has everything to do with air density, meaning that reduced drag needs less power. Of course the engine also has less power available at those jet altitudes, too, again because of reduced air density, and as a result it also burns a lot less fuel for a given TAS.

 

However top speed on an aircraft (not cruise speed) generally happens at or near sea level, primarily because the higher air density lets the engine develop a lot more power (burning a lot more fuel too, of course).

 

One thing that may be confusing you a bit is the mach number (percent of the speed of sound). The speed of sound varies primarily with temperature, so is slower in TAS at jet altitudes than at sea level. The speed of sound at sea level is around 760 MPH, while at 35,000 ft. it's around 660 MPH, because it's so very much colder up there. That's around 660 kts at sea level and around 574 kts at 35,000. So .85 mach at sea level will give you about 645 MPH, while .85 mach at 35,000 ft. will give you about 560 MPH. And at jet altitudes the never exceed speed is given as a mach number for the aircraft. At sea level it's usually CAS (Calibrated airspeed), though we usually think of it as IAS.

 

The above are approximate because the temperatures will vary, but the sea level figure is under the ISA standard conditions (59º F, 0% humidity, 29.92 Hg").

Edited November 20, 2020 by lnuss

Not generally. But at a given TAS it will require a lot less power, which has everything to do with air density, meaning that reduced drag needs less power. Of course the engine also has less power available at those jet altitudes, too, again because of reduced air density, and as a result it also burns a lot less fuel for a given TAS.

 

However top speed on an aircraft (not cruise speed) generally happens at or near sea level, primarily because the higher air density lets the engine develop a lot more power (burning a lot more fuel too, of course).

 

One thing that may be confusing you a bit is the mach number (percent of the speed of sound). The speed of sound varies primarily with temperature, so is slower in TAS at jet altitudes than at sea level. The speed of sound at sea level is around 760 MPH, while at 35,000 ft. it's around 660 MPH, because it's so very much colder up there. That's around 660 kts at sea level and around 574 kts at 35,000. So .85 mach at sea level will give you about 645 MPH, while .85 mach at 35,000 ft. will give you about 560 MPH. And at jet altitudes the never exceed speed is given as a mach number for the aircraft. At sea level it's usually CAS (Calibrated airspeed), though we usually think of it as IAS.

 

The above are approximate because the temperatures will vary, but the sea level figure is under the ISA standard conditions (59º F, 0% humidity, 29.92 Hg").

 

Any credible sim should offer the choice of KIAS or Mach number. Usually the change is made around FL230.