Gear warning horn adjustment

[Jer]

Folks:

On my 89 M20J, the gear warning horn seems to not activate until the MP is down around 11 inches.  This seems a little low to me.  Does anyone know if the trigger point is adjustable, say up to 15 inches.  At what MP does your warning trigger at?

Jerry

[DonMuncy]

The gear warning is widely adjustable. It is not set to MP, but throttle position. There is a micro switch on the throttle cable just behind the panel. 

[Jer]

Don, thank you for that.  While acknowledging that the warning horn is responsive to throttle position, I still would like to hear what MP that you and others experience as the trigger point.

Jer

[amillet]

I set mine to 13”

[EricJ]

Be careful that many of the switches have a fair amount of hysteresis, so if you set it to come on at 15", you may not be able to get it to turn back off until 18-19" or something like that.   This can be annoying if you're just trying to descend, and keep having to add more power than you want to get the horn to shut up.

Every time I think mine is set too low I'm reminded of this. 

 

[PT20J]

It’s really a Micky Mouse arrangement. It takes some fiddling and trial and error and it can also create a hard spot in the movement of the throttle cable where the switch actuator rubs on the ferrel that attaches the cable to the throttle shaft. And you have to wedge yourself into the pilot’s footwell to get to it.  

Where to set it is up to you. The POH says it comes on when the throttle is about a quarter inch open. The service manual says less than 10” MAP. I set mine for 12” MAP.

Mark the throttle shaft with a sharpie in flight when it is set for the manifold pressure where you want the warning to occur. Then on the ground set the throttle to the mark and adjust the switch to just activate. 

[Jer]

All very helpful.  Thank you for the input.

[Jetpilot86]

On 5/27/2024 at 1:05 AM, PT20J said:

It’s really a Micky Mouse arrangement. It takes some fiddling and trial and error and it can also create a hard spot in the movement of the throttle cable where the switch actuator rubs on the ferrel that attaches the cable to the throttle shaft. And you have to wedge yourself into the pilot’s footwell to get to it.  

Where to set it is up to you. The POH says it comes on when the throttle is about a quarter inch open. The service manual says less than 10” MAP. I set mine for 12” MAP.

Mark the throttle shaft with a sharpie in flight when it is set for the manifold pressure where you want the warning to occur. Then on the ground set the throttle to the mark and adjust the switch to just activate. 

At least I know what that hard spot is now. 

[Vance Harral]

Obligatory pedantry from a high-density-altitude resident: manifold pressure at a specific throttle position changes with density altitude.  So if you adjust the switch to trigger at, say, about 12" MP around your sea-level airport, it won't trigger above about 8" MP when approaching an airport that's 4000' higher than your home field.  That means it might never trigger on a normal approach to that 4000' field.

So... if you operate out of both high DA and low DA airports, your options are to have the switch trigger at annoyingly high MP, down low; or potentially not trigger when you need it, up high.  We choose the former because of where our airplane is based, others might understandably choose the latter.

[Rick Junkin]

22 hours ago, Jetpilot86 said:

At least I know what that hard spot is now. 

On your Bravo the switch is forward of the firewall and almost equally hard to get to. But the Bravo gear warning is only referenced to throttle position, so only one variable to be concerned with.

Cheers,
Junkman