Overspeed warning when I am not even over speeding?

[Major Kirrahe]

I'm using a B737 and before I even reach 400 knots, I get an overspeed warning. It just won't go away either. Whether I'm flying level or climbing, I get the overspeed warning! Any input is really appreciated. Thanks!

[BurgerJonas]

What altiude are you flying at when this occurs? Remember, a 737 is not designed to fly at 400 knots at lower altitudes. It seems like you have "display indicated airspeed" checked in the realism settings. Although this is more realistic, you can also check "display true airspeed" and your airspeed indicator will display the ground speed rather than the indicated airspeed.

[freeskier93]

This is correct, maximum indicated air speed (IAS) for a 737 is around 340kts IIRC, or about Mach 0.8 at 35,000 feet. You are probably confusing IAS with ground speed.

[Major Kirrahe]

Oh I see , interesting! I didn't know that the height also depends on it. I just remember reading online about the cruise speed of the 737 being around 420 knots. Thanks both for the answers, it all helps to improve.

[freeskier93]

Originally posted by Lloydy | ロイド:

Oh I see , interesting! I didn't know that the height also depends on it. I just remember reading online about the cruise speed of the 737 being around 420 knots. Thanks both for the answers, it all helps to improve.

Cruise speed typically refers to speed relative the ground. For airplanes this is a rather unimportant value, what an airplane cares about is speed relative to the air since that is what dictates the aerodynamic forces on the airplane, or what the airplane "feels".

Because the air is less dense at cruise altitude 340kts IAS at 35,000 feet will translate into a much higher ground speed then 340kts IAS at 5,000 feet. However, in both cases the airplane "feels" the same thing.

[Thogmar]

Originally posted by JonasBecsan:

What altiude are you flying at when this occurs? Remember, a 737 is not designed to fly at 400 knots at lower altitudes. It seems like you have "display indicated airspeed" checked in the realism settings. Although this is more realistic, you can also check "display true airspeed" and your airspeed indicator will display the ground speed rather than the indicated airspeed.

That is incorrect. Groundspeed is speed over the ground, which is true airspeed plus or minus any winds. Your airspeed indicator NEVER shows your groundspeed, regardless of what setting you tell FSX to use. You can see groundspeed on most GPS units though.
Indicated airspeed is the speed always shown on the basic display of the airspeed indicator, which does NOT show accurate airspeed for higher altitudes which have lower temperature and pressure.
If you select "display true airspeed", then the IAS gauge will show a more accurate AIRSPEED (usually higher than the original indicated airspeed) but it is NOT your GROUNDSPEED, as it doesn't calculate any head/tailwind you are flying in.

[Thogmar]

Originally posted by freeskier93:

Originally posted by Lloydy | ロイド:

Oh I see , interesting! I didn't know that the height also depends on it. I just remember reading online about the cruise speed of the 737 being around 420 knots. Thanks both for the answers, it all helps to improve.

Cruise speed typically refers to speed relative the ground. For airplanes this is a rather unimportant value, what an airplane cares about is speed relative to the air since that is what dictates the aerodynamic forces on the airplane, or what the airplane "feels".

Because the air is less dense at cruise altitude 340kts IAS at 35,000 feet will translate into a much higher ground speed then 340kts IAS at 5,000 feet. However, in both cases the airplane "feels" the same thing.

No and no:

1. Cruise speed has nothing to do with ground speed they are always airspeed related, and they are speed PERFORMANCE guidelines or limits, not a speed measurement from an instrument. They are a guideline to set your airspeed to for various reasons. I.E. Mach .72, or 350 knots, or 180 mph, or whatever value is desired at the time by ATC, or by the pilot for whatever reason they wish - spacing, fuel economy etc. Think of them like the speed limit signs on the highway, they don't tell you how fast you are going, they are numbers that tell you how fast you should be going (typically in a chart) and you adjust your speed to them.

2. Air density and pressure differences affect TRUE AIRSPEED, which of course is a component of groundspeed, but not the entire package which includes any head or tailwinds. And no, it's not a guarantee of a higher GROUNDSPEED, since a strong headwind at altitude could actually make for a SLOWER groundspeed at high altitude. That's why it's very important to know the differences between indicated/true/groundspeeds.

And aircraft don't "feel" anything, that is an abstract of no use to a pilot.

[Thogmar]

Originally posted by Lloydy | ロイド:

I'm using a B737 and before I even reach 400 knots, I get an overspeed warning. It just won't go away either. Whether I'm flying level or climbing, I get the overspeed warning! Any input is really appreciated. Thanks!

What people here are miserably trying to tell you is that your TRUE airspeed at high altitude is actually higher (say 450 knots) that what your INDICATED AIRSPEED GAUGE is showing you. Therefore you can be overspeeding even though the gauge doesn't seem to show it.

For that reason, some airspeed indicators have a separate set of windows that let you set and display a TRUE airspeed that you adjust for temperature and pressure settings that you determine from other instruments or information sources. THIS is what you can simulate in FSX by selecting the "display true airspeed" setting if you desire.

This is also why you should be looking at your MACH meter and know what your airspeed limitations are in MACH, as that gauge is designed to work with an air data computer to show you an airspeed adjusted for current temperature and pressure for the current altitude. I.E. basically a TRUE AIRSPEED, but even more accurate as it also calcuates in the varying compressibility of air at different altitudes.

Ignore GROUNDSPEED for any of this issue as it is irrelevent. It is important for other reasons, but has nothing to do with OVERSPEED issues.

[Thogmar]

Originally posted by Lloydy | ロイド:

Oh I see , interesting! I didn't know that the height also depends on it. I just remember reading online about the cruise speed of the 737 being around 420 knots. Thanks both for the answers, it all helps to improve.

Note that a "cruise speed" is just an optimal speed setting for a desired parameter (good fuel economy, maximum speed, etc.) It's not the same as the "overspeed" limit which relates to structural and engine safety limits.