I have a few questions about speed hold on heavy jet. I found that there are two ways to hold the speed - airspeed hold and march hold. What are the differences between them?
Indicated Air Speed (IAS) is most useful at lower altitudes, say below 26,000 feet. IAS is a measure of the stress on your aircraft due to the air you are flying through. At high altitude, the airspeed indicator becomes very inaccurate, because the air is so thin.
At high altitude, Mach (not march) number is most useful. It is your speed relative to the speed of sound at whatever altitude you're flying. The Mach number is a ratio of your speed to the speed of sound, so for example, Mach .81 is 81% of the speed of sound at your altitude. Exceeding the speed of sound would put a great stress on the airframe, and passenger jets are not designed to do that (except Concorde).
So use IAS down low, Mach number up high.
Mach 1 in Concorde is barely second gear! 😂