When is the right time to switch from Knots to Mach?

Pro Member First Officer
leachus2002 First Officer


I have been flying for about a year now, but I have never really used MACH when flying at high altitudes. I have only just started using it.

I have noticed also that when I use FSpassengers, my co pilot announces at about 18.000ft...."Passing transition altitude".

I have always understood this as the transition from ALPHA airspace to BRAVO airspace.

Is this correct? Also, is this a good time to switch to MACH from Knots?


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Pro Member First Officer
Tartanaviation First Officer

Transition altitude at 18000ft is when aircraft flying at or above that altitude set their altimeter to 29.92.

Here is some information from a site i found

On the Boeing 777, below 31,000 feet, the airspeed is referenced by IAS (indicated airspeed) in knots or nautical miles per hour. Above 31,000 feet, the IAS automatically changes to Mach number in the airspeed indicators.

Why Mach numbers are used? Well, in aerodynamics, when an aircraft 'breaks the sound barrier', it is said to have reached Mach 1. When a airplane is capable of exceeding the speed of sound, there are other implications associated with it. The airplane now becomes supersonic and shock waves begin to form on the wings, and drag rise significantly. This is a phenomenon that airplane manufacturers try very hard to prevent. Now you know why all the airliners, including the latest Airbus 380, have been designed to cruise at less than Mach 0.9 for fear of hitting the 'shock waves' and burning more fuel!

If airspeeds are not referenced to Mach numbers, it would not be possible for the pilot to be warned that his airplane is approaching the speed of sound or hitting the shock waves (especially in a dive).

Pro Member Chief Captain
CRJCapt Chief Captain

Your FO or co-pilot, says 18000 ft. because this is the Transition altitude between using the local altimeter setting(QNE) and the standard setting above FL180 of 29.92 in or 1013 mb (QNH). Almost all jets are still climbing at an indicated airspeed(IAS) as they go above FL180. As you climb, IAS will decrease and Mach number will increase. Most jets have a climb schedule that says: 250 KIAS below 10000' 300 KIAS until reaching Mach .7. This will occur at approx. FL 280. It's not a precise altitude so don't hold me to it. Then cruise depends on aircraft and length of flight. Mach .74 to .85 depending on how much fuel you want to burn. By the way. Jets are fastest in the FL280-FL330 altitudes. Above that they lose speed but burn less fuel.

Pro Member Chief Captain
Tailhook Chief Captain

CRJCapt wrote:

...By the way. Jets are fastest in the FL280-FL330 altitudes. Above that they lose speed but burn less fuel.

Thanks for that. I'd been wondering why with some airlines on relatively short hops the a/c climbs to FL390.

Pro Member Captain
Ian Stephens (ianstephens) Captain
Ian Stephens is an expert on this topic. Read his bio here.

Hello Leachus,

Firstly, I'd like to clarify that the term "transition altitude" does not refer to a change from Alpha to Bravo airspace. Transition altitude (TA) is the height above Mean Sea Level (MSL) where pilots transition from flying at an altitude expressed in feet to flying at a Flight Level (FL) using a standard altimeter setting of 29.92 inHg (1013.25 hPa). The actual value for transition altitude may vary depending on the region, and in the US, it is typically 18,000 ft.

Now, to answer your question about when to switch from knots to Mach, this is usually done when an aircraft reaches its crossover altitude (CA). The crossover altitude is the point where the aircraft's True Airspeed (TAS) is equal to its Mach number. This typically occurs at a specific altitude, which varies depending on the aircraft and atmospheric conditions.

For most commercial airliners, the crossover altitude is generally between 25,000 ft and 30,000 ft. While the exact altitude may differ, you can use this range as a guideline. When your aircraft reaches this altitude range, you can switch from using knots (Indicated Airspeed or IAS) to Mach.

To summarize, the transition altitude is not the ideal point to switch from knots to Mach, but rather the crossover altitude. As a rule of thumb, the crossover altitude for most airliners is around 25,000 to 30,000 ft. Keep in mind that these values may differ depending on the aircraft you are flying and the prevailing atmospheric conditions.

I hope this clears up any confusion. If you have any further questions, feel free to ask!

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