G'day,
In turbo props the decreasing limit with altitude is usually caused by the speed of the prop tips nearing the speed of sound. As the prop tip speed gets close to the speed of sound the prop becomes less efficient.
One thing to remember:
The speed of sound decreses as temperature decreases.
Let's say we are doing 224 knots IAS at Sea Level on an ISA day - this equates to approx. Mach 0.339
Let's then say we are doing 224 knots IAS at 20,000' on an ISA day (-25 deg C) - this equates to approx. Mach 0.492
The Fokker 50 manual doesn't give a specific maximum Mmo limit (Well mine doesn't!) but does say that you can maintain 224 knots up to approx 20,000 feet. This would tend to indicate that the Mmo limit is around mach 0.492 on the Fokker 50.
If we were to then do 224 knots IAS at 25,000' on an ISA day (-35 deg C) this would equate to Mach 0.544
As you can see this now exceeds the MMo limit. Consequently, to stay under the Mmo limit we need to fly at a slower speed.
The 'barbers pole' indicates the most limiting figure of Vmo or MMo - in the Fokker 50 this would be 224 knots up until approx. 20,000 feet and then the barbers pole starts moving back on the airspeed indicator until it gets back to approx 206 knots at 25,000' (Max ALT for the F50)
One thing to note on the Fokker 50 is that if you turn the Prop electronic control system off then the prop RPM will increase from 85% (in cruise) to 104%. This increases the tip speed of the props significantly and puts them closer to the speed of sound - to compensate for this there is a table of reduced speeds in the QRH to fly if this happens.
As a matter of interest the Max IAS at 20,000 feet goes from 224 knots with the prop control working down to 161 knots if it fails - quite a difference!