Silly "Overspeed" Alarm

[Mac6737]

Last week, I had to quit an airbus 320 flight, because I couldn't get the overspeed alarm to turn off. (Never had this problem before.) For other reasons I installed the FBW 320 a few days ago. The AP seems less quirky, but last night I got the overspeed warning again. As you see from the screenshot, I was at FL 320, the red stripes on the speed dial start at around 320 kts., while my indicated speed is only about 300. At that altitude, you shouldn't be limited to 320, let alone 300. (BTW, my flaps were all the way up.) I was able to get it to stop by reducing speed to 285, but even so . . . .

 

Am I doing something wrong, or is this a known bug?

 

 

[lnuss]

I don't have that sim or that aircraft, but at that altitude the warning is likely to be from exceeding a mach number, rather than IAS. I didn't spot a mach meter in your screenshot, but it's something you might explore. Or maybe it IS a bug...

[tiger1962]

You got the Overspeed warning because you were over speed by quite a margin - the figure you should be looking at is below the speed tape in green. This is the Mach number. The cruise speed for the A320 is .78 (of the speed of sound) and you were doing .839! Mach .78 equates to around 280-290 knots indicated on the speed tape, depending on headwinds of course. You can toggle the speed selector knob between mach and knots by clicking on it, you'll want to switch from knots to mach when climbing thru FL180 or Transition Altitude, and back again when descending. Edited December 10, 2021 by tiger1962

[mallcott]

Last week, I had to quit an airbus 320 flight, because I couldn't get the overspeed alarm to turn off. (Never had this problem before.) For other reasons I installed the FBW 320 a few days ago. The AP seems less quirky, but last night I got the overspeed warning again. As you see from the screenshot, I was at FL 320, the red stripes on the speed dial start at around 320 kts., while my indicated speed is only about 300. At that altitude, you shouldn't be limited to 320, let alone 300. (BTW, my flaps were all the way up.) I was able to get it to stop by reducing speed to 285, but even so . . . .

 

Am I doing something wrong, or is this a known bug?

 

 

[ATTACH=CONFIG]228207[/ATTACH]

 

It's YOU, not the plane. Read up on IAS v. altitude....

YOU should be setting Transition Altitude no later than 23K, and preferably lower.

[plainsman]

Like Larry mentioned, as you pass through 17,500 feet, get your instruments ready to switch speed from knots to Mach by 18,000 feet. Then stay within the Mach limits of the aircraft and you won't get the warning.

[mallcott]

Like Larry mentioned, as you pass through 17,500 feet, get your instruments ready to switch speed from knots to Mach by 18,000 feet. Then stay within the Mach limits of the aircraft and you won't get the warning.

 

Here in the UK, we can experience Transition Altitude as low as 4,000 ft ASL, common at 6,000... Shame this new sim can't do this. The French also have a far lower TA - commonly 5,000 ft ASL.

[Mac6737]

Happy to hear it's not a bug. But saddened that I really don't understand something:

 

You guys tell me to switch from knots to mach, but I thought the mach number is merely the speed in knots, restated as a percentage of the speed of sound. So why should the "overspeed" monitor care whether your speed is stated as knots or mach?

 

Also, you refer to the mach limit of the aircraft, and one of you says the mach limit for the 320 is .78, or about 290 knots. Okay, so then the red lines on the speed tape should also start at 290, no?

 

Finally, are you telling me that the mach limit of the 320 is .78, period? -- meaning a 320 can't safely go faster than 290 knots? Then why are there all those higher numbers on the speed tape? (Okay, the speedometer on my 1960 Austin-Healey 3000 went up to 120 mph, but the car sure wouldn't.) Anyway, I've exceeded 290 knots in the 320 many times without getting an overspeed warning.

[lnuss]

You guys tell me to switch from knots to mach, but I thought the mach number is merely the speed in knots, restated as a percentage of the speed of sound. So why should the "overspeed" monitor care whether your speed is stated as knots or mach?

 

Apparently you actually don't understand about mach number. The speed of sound itself changes with (mostly) temperature and, given ISA standard conditions the sea level speed of sound is about 760 mph and at 35,000 ft is around 660 mph, or so. Temperature decreases as altitude increases, generally speaking, so the speed of sound decreases at higher altitudes (because of temperature differences, of course).

 

Mach number is a PERCENTAGE of the speed of sound, so that when mach 1 is 760 mph at sea level then .78 mach is 592 mph, or 515 kts TRUE airspeed (not indicated). At the same time (again under ISA standard conditions) mach 1 at 35,000 ft. is 660 mph so that mach .78 is 514 mph or 447 mph TAS (not IAS).

 

An airframe has various limitations placed on it because of various pressures and loads that it encounters under different conditions. So there are limitations in IAS that mostly define wing behavior and such, including stall speeds, Vne (Never Exceed Velocity), climb rates etc. And that Vne is generally because of pressures up to some point.

 

However, as an aircraft approaches the speed of sound under existing conditions (that is, what reads on a mach meter), there are other types of pressures placed on the airframe, often (not always) due to the potential for shock waves, etc. that are due to being close to the speed of sound. These are DIFFERENT problems from those due to exceeding Vne, though exceeding either set of limitations can destroy the aircraft.

 

So there is NO DIRECT correlation between IAS, TAS and mach that always holds true, thus dictating that as you get into an area where the speed of sound is reduced (that can be at lower altitudes in the arctic, also, due to temperature) then the limitations are more restricted by mach than by IAS, which gets lower with altitude (if that's a mystery read up on density altitude, IAS vs TAS, etc.). However the computer associated with the mach meter computes all of that so that all you have to do is read it's calculation, that is, the meter.

 

Thus, in light of the above, getting above roughly 18-20K feet (FL18 -FL20 and above) it's prudent (and a required procedure in real life) to switch to using the mach meter.

 

Finally, are you telling me that the mach limit of the 320 is .78, period?

Yes!

 

-- meaning a 320 can't safely go faster than 290 knots?

No, it doesn't mean that. At lower altitudes you can go up to the red line on the airspeed indicator. You apparently don't understand the difference between INDICATED airspeed (which is basically a pressure value) and TRUE airspeed which is an ACTUAL speed value through the air (nothing to do with the ground -- GROUNDSPEED is another can of worms). So at Jet altitudes that 290 kts is over 400 kts, perhaps approaching 500 kts TRUE airspeed (I didn't try to calculate it, not know ALL conditions you are seeing).

 

Okay, so then the red lines on the speed tape should also start at 290, no?

No! That is for defining Vne, and is correctly marked. Hopefully it's more clear now that you have read the above.

 

Many of these terms are very well explained in their respective Wikipedia entries, so you might check those out. While you're at it, you might want to learn about pressure altitude, density altitude, true altitude, absolute altitude and such. The variations in air density with barometric pressure, temperature, altitude, etc. can also be quite confusing, and once you understand the various different ways that speeds and altitudes are measured and used in aviation, the less confused you will be.

 

Much of aviation is FAR from intuitive, and is more so at higher (jet) altitudes, too, and is one (of many) reasons that real life pilots need special training (in addition to everything else) JUST for flying in the Flight Levels.

 

There's a LOT to learn in order to understand all this. Hopefully this helps.

Edited December 11, 2021 by lnuss

[Mac6737]

TG,

 

Thanks so much for that exegesis. It is most helpful. (Actually, I did know that Mach 1 changes with ALT, but that's about all.) So next I'll have to find out what the mach limit is for planes other than the 320. I'm sure there's a way.

 

What still remains a mystery to me is how I've been playing around with flightsim since FS 2.0 on an Apple IIc in the early 90s, but had never before encountered this can of worms.

[plainsman]

Mac, there are several things to think about in this regard. First is the structural consideration of max Mach limit. Second is the economic Mach limit which determines actual operating speeds.

On the structural side, some examples of Mach limits aside from the A320, the 737-300/400/500 had a limit of Mach 0.82 but operated below Mach 0.76. The 737-600/700/800/900 had a Mach limit of 0.785. The ERJ-195 has a Mach limit of 0.82. The 747-100/200 had Mach limits of 0.92, but operational limits of 0.84. The 747-300/400 were also Mach 0.92 limit, but operationally limited to 0.85. The 747-800 has a Mach limit of 0.900, but an operational limit of Mach 0.855.

The reason for the operational limit is largely economic. As the speed approaches Mach 1.0, pressure builds up in front of the wings. This creates drag. It takes LOTS of fuel to overcome that drag. For example, an old Convair CV990A burned about 14,000 pounds of fuel per hour at Mach 0.85 at 35,000 feet. A 737-700 (about the same passenger load), would burn about 5000 pounds of fuel at Mach 0.785 and 35,000 feet in the same time. The A320Neo will reduce that fuel burn even more. Also, since that added pressure (drag) stresses the airframe more, the life of the aircraft is reduced at Mach limits. Another limiting factor is the operational limits of the jet engines. Since a huge amount of thrust is needed to overcome the added drag, some of the operational limits may be due to limiting N1 and N2. Thus most commercial airliners operate below their design limits.

Edited December 12, 2021 by plainsman

[Mac6737]

OK, so I guess the bottom line for MSFS pilots is:

 

If you get the warning, slow down!

[lnuss]

OK, so I guess the bottom line for MSFS pilots is:

 

If you get the warning, slow down!

 

Well, yes, but better is to learn to notice the limitation markings on the airspeed and mach meter (and to use the correct one), and to therefore avoid the overspeed in the first place.

 

Enjoy!

[Cavulife]

Well, yes, but better is to learn to notice the limitation markings on the airspeed and mach meter (and to use the correct one), and to therefore avoid the overspeed in the first place.

 

Enjoy!

 

I’m not the OP, but this still confuses me. In his picture it shows that his airspeed is below the warning area so it seems that he shouldn’t get the overspeed warning?

[plainsman]

You are making the same mistake. His pic shows a Mach reading of 0.839, way over the Mach 0.78 limit for the Airbus. The speed in knots is somewhat irrelevant in that at 17-18,000 feet and above, he should be using a Mach speed to define the acceptable performance envelope.

In other words, there are multiple constraints. He is limited to an IAS but also by a do not exceed Mach 0.78. Which ever is applicable must be honored. If he were at 2,000 feet the IAS would be the limiting factor. At 18,000 feet Mach is the limiting factor. Each limit is a function of different stresses the aircraft can sustain. Read all the comments carefully and that will become clear.

Edited December 13, 2021 by plainsman

[lnuss]

I’m not the OP, but this still confuses me. In his picture it shows that his airspeed is below the warning area so it seems that he shouldn’t get the overspeed warning?

 

If you will read some of the explanations above it should become more clear -- that's why the explanations are there. The differences between TAS, IAS, mach, which airspeed indication to read and more is explained above, but basically he should have been using the mach meter, not the airspeed indicator, since different conditions result in different limitations.