When does the overspeed alarm goes off in a plane?

Question

I want to know when does the overspeed goes off in aircraft. How exactly is overspeed limit decided? Is it fixed for all the situations, or does it keeps changing? In a game that I play, (Infinite Flight) it goes off at 350 knots air speed for a Bombardier CRJ 200 or Cessna Citation X. But, it seems unrealistic as real planes often easily fly over 450 knots air speed.

Answer 1

The overspeed goes off when either the indicated airspeed exceeds that limit or the mach speed exceeds the mach limit. At least that's how it worked on 747-100/200 aircraft.

The 350 knot figure for the max operating airspeed sounds reasonable to me as long as you're low. From memory that was around the max operating INDICATED airspeed for 747-100/200 aircraft. Remember, though, that indicated airspeed decreases with air density. A 747 cruising above 30,000 feet at around 550 knots TRUE airspeed is going to show an indicated airspeed of less than 300 knots. However, that cruising true airspeed of 550 knots or so is going to be around mach 0.86. The max mach number for the 747 was 0.92.

If you were at, say, 15,000 feet and speeded up to 350 knots, the mach number would be much less than 0.86.

Take your sim up to 35,000 and see what you get.

Answer 2

The overspeed clacker on the CRJ 200 series will sound "at" a calculated limit speed which is either VMO or MMO (depending on altitude) according to the documentation available at smartcockpit.com . The Cessna Citation X is similarly limited "at" VMO or MMO. You can find these numbers on other answers.

One thing to keep in mind is the difference between true air speed and indicated airspeed/calibrated airspeed. At a constant indicated air speed, the true air speed (the speed with which you're physically moving through the air) will be higher at higher altitudes and higher temperatures. However, the speed limits are aerodynamic numbers and are given as indicated airspeed numbers, because despite changes in altitude and pressure the limits stay at the same indicated air speed. Therefore, if VMO is 350 knots you can go faster than 350 knots true airspeed, but only at a high enough altitude where your indicated airspeed is still lower than 350 knots. For example, at 20,000 ft. MSL I can go 450 kts. true airspeed without going above 350 knots indicated airspeed.

VMO is used in place of MMO when VMO is lower. When the airplane is at a low enough altitude, a constant IAS is used for the limit and at higher altitudes the calculation is done instead in mach, as shown in the diagram below. Note that the horizontal axis is in knots indicated airspeed, not Mach number or true airspeed.

Sample VMO vs MMO chart from "Aircraft Trajectory Tracking by Nonlinear Spatial Inversion" at Research Gate

Now, I say "at VMO" in quotation marks because often this calculated limit speed is offset slightly above the VMO/MMO so you can fly at exactly the VMO/MMO without the alarm going off. (See this Aviation Today article for examples from Airbus). There is also sometimes logic to detect and warn about imminent overspeed events. When accelerating rapidly, these warnings will go off slightly before the normal VMO/MMO alarm would. The Cessna Citation X has similar behavior, but it does not seem to affect the overspeed warning horn: "When the airspeed trend vector exceeds VMO by one knot, the rolling digits turn amber unless a red indication is called for."

Many planes also have a similar speed where an overspeed protection feature in the flight guidance system activates (similar to stall-preventing underspeed-protection modes), but I couldn't find anything listing such a feature for the CRJ 200 series. There is a warning saying that some modes can command the CRJ 200 to exceed VMO/MMO if used improperly, so I don't think it uses VMO/MMO to limit flight director commands. The Cessna Citation X flight guidance system does not behave like the the CRJ 200, and will limit commands "to VMO/MMO" according to the manual for the Citation X Model 750 at smartcockpit.com So the Citation X does have a speed protection feature, but according to the documentation I can find this also activates "at VMO/MMO."

Note that for both jets this speed is dynamically calculated by the avionics based on configuration (extending flaps and lowering landing gear decrease the VMO). For example, the Cessna Citation X has the following VMO speeds listed for the Model 750:

╔═════════════╦═════════════╗
║   Config    ║ Speed Limit ║
╠═════════════╬═════════════╣
║ Flaps 5°    ║ 250 Knots   ║
║ Flaps 15°   ║ 210 Knots   ║
║ Flaps > 15° ║ 180 Knots   ║
║ Gear Down   ║ 210 Knots   ║
╚═════════════╩═════════════╝

Answer 3

I can't find a V_NE for them but a post on Airliners.net lists the V_MO, which is the fastest you're supposed to fly it in cruise, for the CRJ200 as:

• 0 - 8000ft 330kts

• 8000ft - FL255 335kts

• FL255 - FL280 M0.80

• FL280 - FL316 315kts

• FL316 - FL410 M0.85

This AOPA article lists the V_MO of the Citation X as Mach 0.92, or 547 KTAS. The corresponding IAS would depend on altitude and temperature. From the same article the updated version, called the Citation X+ the speeds are listed slightly higher:

• V_MO (max operating speed) at sea level to 8,000 ft | 270 KIAS

• V_MO (max operating speed) at 8,000 ft to 30,650 ft | 350 KIAS

• M_MO (max Mach number) | 0.935 M*

* At 30,650 0.935M would be 549 KTAS

Answer 4

I would like to add a bit of explanation for why the rules for $V_{MO}$ are so complex:

There are three different phenomena that limit the maximum speed of an aircraft:

1. The direct pressure peeks and minima that stress the structure. These are proportional to the dynamic pressure, which is measured as indicated airspeed (well, equivalent airspeed). At certain indicated speed, the pressures will be more than the structure was built for and you risk damage, so there is maximum operating speed, $V_{MO}$.

   Indicated airspeed is proportional to the air density and square of speed. At sea level it is numerically equal to the true airspeed, but high up in the typical cruise levels you can have 460 knots true with just, say, 270 knots indicated and Mach 0.80 (the actual values depend on altitude and on pressure and temperature on the day).

2. Every structure is to some extent flexible and has a resonant frequency. The turbulence around the structure creates pressure oscillation with its own frequency and this frequency decreases with speed. At some speed, the frequency will match the resonant frequency of the structure, which will cause aeroelastic flutter.

   Since the frequencies depend on true airspeed, there is a maximum for that as well. But because instruments in aircraft only show indicated airspeed, there is instead a table or graph showing decreasing $V_{MO}$ with altitude.

   In modern aircraft, the table is built into the flight management system, so the $V_{MO}$ for current altitude is shown on the airspeed indicator and the alarm is sounded when it is exceeded.

3. When the flow around the wing exceeds the speed of sound, the pressure field changes, shifting the centre of lift aft, which causes pronounced pitch-down moment. Aircraft not designed for supersonic flight may not have enough elevator authority to compensate for this, so a loss of control might result.

   This depends on Mach number (ratio of true airspeed to speed of sound). Since speed of sound only depends on temperature, for aircraft that fly in transsonic regime, a $M_{MO}$ is usually selected so that it covers both this and flutter at temperatures that can be expected at the cruise altitudes, simplifying the logic a bit—you only have a $V_{MO}$ and $M_{MO}$ and up to some altitude, you reach $V_{MO}$ first and above it you reach $M_{MO}$ first.

Additionally, there is usually $V_{MO}$/$M_{MO}$, maximum operating speed, and slightly higher $V_{NE}$/$M_{NE}$, never exceed speed. This is mainly about safety margin. The maximum operating speed is what you should stick to. If you reach the never exceed speed, your safety margins are getting really thin.