# Why is the normal operating airspeed so much lower than the cruise speed in this table?

I need to work with the following table. I'm interested in vMO which I assume I can simply obtain from converting MMO to vMO through $$M = v/ \sqrt{1.4 * 287 * T}$$. (Is that correct?)

However, I noticed that the vNO is more than 100 kts less than vcr in this table. From my current understanding, vcr should be given as a TAS, which would mean that the the only way in which vNO could be lower than vcr would be that vNO is given in IAS. However, in that case the difference should be much more than 100 kts, right? • Regarding your first part: $V = M \cdot a$, $a = \sqrt{\gamma \cdot R \cdot T}$ with $R = 287.05$. That computation is correct. Considering the table: are you sure these speeds are at the same altitude? Similarly: do you have a clarification of what the author of this means with the symbols? – Bram Nov 17 '18 at 11:34
• Unfortunately I don't know for sure at which altitude vNO/MMO are specified, neither do I know what the symbols exactly mean, but I'm assuming that these are used according to the standard definitions since the author doesn't provide any other information. We're supposed to use this table for an assignment, but for now it's causing a lot of confusion. – Daniel Nov 17 '18 at 11:38

VMO and MMO are fix values independent of altitude. VMO is given in IAS. You can convert MMO to an airspeed based on altitude, but really they are two independent limits, and whichever limit is lower at your altitude must be observed.

Your max cruise speed data only makes sense if a TAS; converting it to a Mach number at the given cruise altitude reveals it is the same as MMO only expressed in TAS at given altitude.

• Is vNO given in KIAS as well? – Daniel Nov 19 '18 at 16:11
• @Daniel It has the same values as VMO, so yes. It is normally referred to as VMO as far as I‘m aware. – Cpt Reynolds Nov 19 '18 at 16:46

You really answered your own question. Vne is based on IAS, the actual aerodynamic pressure at any given altitude and speed. This is why the Mach number is included too, as approaching transsonic adds additional stresses to the airframe.

TAS, as mentioned in previous threads will be much higher than IAS at 35000 feet. It is fun to enter speeds and altitudes into the TAS calculator to see how close one gets to Mach 1 at 250 IAS when you climb.

TAS is groundspeed without wind speed and direction factored in. It is the actual aircraft airspeed relative to the airmass. Notice 480 is Max Cruise.

• The numbers don't really match up though. For instance, if I enter 31000ft, 1013.25 hPa,-60°C and 335 kts in here dauntless-soft.com/products/Freebies/TrueAirspeedCalculator, then I get 541 kts, which considerably higher than the 480 kts vcr. But maybe that's just due bad quality of the data? – Daniel Nov 17 '18 at 12:59
• Also, to make sure, vNO and vMO are also given in IAS? – Daniel Nov 17 '18 at 13:00
• So, you can cruise at a lower IAS and save some fuel, no? – Robert DiGiovanni Nov 17 '18 at 13:01
• That's what going high does for you, as long as you stay within subsonic limit. – Robert DiGiovanni Nov 17 '18 at 13:05
• "So, you can cruise at a lower IAS and save some fuel, no?" So basically you are saying that if one follows the vNO restriction at this altitude, vcr_max is exceeded? So basically at cruise the limiting factor is the max cruise speed instead of vNO? – Daniel Nov 17 '18 at 13:08