Metro 23 application of full reverse

[ARNTU]

Can someone explain why full reverse in the Metro (TPE331-12) is restricted to a certain speed during landing. I know it depends on ambient temperature, but why??

[welliewanger]

I have no idea about these aircraft so can only speculate. I'm guessing that there's a minimum speed for full reverse. If so, maybe below this speed the airflow into the engine is too turbulent leading to compressor stall. And / or the usual issues of kicking debris up into intakes.

[rigpiggy]

I don't have the manual in front of me but I think full reverse is not supposed to be selected above 90 kts, due to resonance problems I believe. Years back I was Dh'ing on a Metro2 and the PIC slammed the a/c on about 1/2 way down a 4000' strip. teh engine spinners jumped up/down at least 4 ". Scared the cr@p out of me

[grade-3]

Hi,

When flying the metro I believed that the 90kt limitation was related to bog down and air loads on the prop.

Put briefly, in beta/reverse the power lever controls propellor blade angle, so at a higher forward speed full reverse will create more prop torque (drag) for the engine to overcome. Above 90 KIAS I believe that this extra drag can cause engine RPM to decay below the minimum for reverse (98%?), leading the FCU to add more fuel to maintain an on-speed condition. In some circumstances this extra fuel is still not sufficient and the RPM decays further, so the FCU adds more fuel ... end result is an overtemp and usually really nasty engine damage :{

(This also explains why the 90 kt limit reduces with increased OAT; less engine power, so RPM decay at full reverse will occur at a lower prop torque/drag/KIAS.)

We used to have a restriction to not apply reverse with speeds low for the same reason, and always had to be very careful when bringing the speed levers from high to low after landing that reverse had been cancelled first.

That said, there's no restriction on applying partial reverse above 90 KIAS, and used to find applying a bit of reverse (maybe 1/4 or so) was useful for slowing the aeroplane quickly from the higher VREFs of a heavy metro 23 (115 KIAS or so).

Regarding FOD, usually find that the props start kicking stuff up on the runway at a fast taxi speed (30-50 KIAS), but never aware of it being a problem at high speed. There's no minimum speed for full reverse that I'm aware of, except that dictated by airmanship.

grade 3