Google doesn't seem to have much.

 

Is it simply a redundancy thing so if one fails the other would be available to use? Just guessing.

No... one holds an indicated airspeed... the other holds an indicated mach speed.

 

At around 36,000ft indicated isn't a safe speed to track... you should be using mach speeds at that point.

Based on my observations;

 

1. Airspeed hold is just that, plug in an airspeed value and the autopilot holds it, regardless of altitude.

2. Mach hold holds a particular mach number to which the associated airspeed value varies with altitude.

 

As the altitude rises (aircraft climbing into the FLs), the airspeed value associated with a set mach number gets smaller, and the autopilot will automatically adjust the airspeed setting to correspond with the mach number at the given altitude.

 

If following just an airspeed setting, as the aircraft climbs, the maximum operating speed of the aircraft can be exceeded causing at the least an "overspeed" warning and at the most catastrophic damage to the aircraft (if you have that sort of thing set up in your preferences).

 

Keep em flying.

Joe

When flying jet aircraft, once passing 18,000 feet and setting your barometric

 

pressure to 29.92,it is a good thing to set your speed control to MACH instead

 

of IAS.  As you climb to your cruise altitude and then leveling out, you can

 

increase MACH to a desired point keeping an eye on the indicated airspeed

 

so as not to go into an OVERSPEED condition.  While flying at MACH you

 

will notice that the KIAS will decrease and ground speed will increase. 

 

When using the auto throttle, the Airspeed hold changes to MACH Hold

 

when you press the MACH button.  Always be aware of wind condition at

 

whatever altitude you are flying.  Make sure your PITOT Heat is on.

When flying jet aircraft, once passing 18,000 feet and setting your barometric

 

pressure to 29.92,it is a good thing to set your speed control to MACH instead

 

of IAS.

18,000 is far too low an altitude to be using mach hold. Most complex autopilots don't switch between IAS and Mach hold before 32,000ft.

When flying jet aircraft, once passing 18,000 feet and setting your barometric

 

pressure to 29.92,it is a good thing to set your speed control to MACH instead

 

of IAS. As you climb to your cruise altitude and then leveling out, you can

 

increase MACH to a desired point keeping an eye on the indicated airspeed

 

so as not to go into an OVERSPEED condition. While flying at MACH you

 

will notice that the KIAS will decrease and ground speed will increase.

 

When using the auto throttle, the Airspeed hold changes to MACH Hold

 

when you press the MACH button. Always be aware of wind condition at

 

whatever altitude you are flying. Make sure your PITOT Heat is on.

Incorrect, switch to 29.92 at the transition altitude which varies greatly country to country.

 

When climbing use IAS hold, stay in IAS hold until you reach "Mach crossover altitude". This is normally done automatically by the AP but if you want to do it manually, a good practice is to switch to Mach hold when the IAS matches your planned cruise Mach speed, this would typically happen above 25'000ft.

So within FSX, do Mach and Airspeed hold work properly as they should or are they both 'dumbed down' simply maintaining a set speed?

They both work correctly, simply adjusting the throttles to hold the speed depending on the attitude of the aircraft. If you pitch too high or too low where by the speed can not be maintained or you can't slow down anymore then that's your problem!

 

On a more complex aircraft, the aircrafts electronics set the engine thrust to a certain level for each stage of the flight, so in the climb stage, the thrust is set and then the aircrafts pitch attitude changes to hold the speed, the IAS is held until Mach crossover then it automatically switches to hold Mach.  You need to use Mach at high levels (cruise levels) due to changes in temp, pressure etc.

The difference between Mach Number and airspeed is essentially that Airspeed is calculated from air pressure velocity and temperature, whilst the Mach no calculation also adds  air density to the mix. Maximum Mach number limit for an aircraft is set to prevent shock wave effects over the aerodynamic surfaces from affecting the aircraft's flight stability.