Cold wx tach error?

[unclenobby]

Wondering if anyone has experienced this or can shed any light on this. Was flying a 1979 Archer II today.

Temp was -8 celcius, Pressure 30 45

During run up at 2000 RPM drop on both mags was a little over 100 rpm (even though max allowable is 125) so I decided to treat as fouled and inc RPM to clear. RPM would not go over 2100 even at full throttle. Engine did not sound rough so I continued and after a minute or so back down to 2000 to check the mags. Drop was significantly less so happy with that result. Continued with the rest of the checklist including carb heat and no sign of ice. Before take off checked full pwr again and still only got up to 2100 - applied carb heat at full throttle and no indication if ice again.

Took the rwy to see if problem would persist during take off roll. Decided on short field take off - full throttle on the brakes again 2100, on the roll RPM only increased another 100 RPM. It felt like T/O pwr and sounded right but aborted T/O and headed back in (usually 2550 RPM for T/O)

Spoke with the mechanic and explained what happened. He took it out for a good run up and got the same results. Since he is not a pilot I went out with him again to show him the tach on the roll (his was just a static check). Same again, max 2200 RPM even though T/O pwr felt good and engine sounded good. Brought it back and called it a day. Mechanic seemed unsure of what was wrong.

I have flown the a/c a lot and flew in cold wx also and first time seeing this. While I was there the owner of the school called and was talking to dispatch....he told dispatch that the aircraft has a hydraulic tach (??) and the cold wx may have affected the instrument (I got this second hand so may not be verbatum). I did not get preheat before start up but by the time the mechanic and I were finished with all our checks the engine had 0.8 on the tach so it would have been warm during the final aborted T/O roll.

I'll stress there were no issues during mag check and no sign of ice - applied carb heat for 30/60 seconds and no increase in RPM after initial decrease. Also, temp/dep point spread was about 15 degress celcius so it was very dry.

I'm curious to find out if anyone have had a similar experience or if anyone has any ideas as to what occured?

Thanks

 

[Berkut]

I've never heard of a hydraulic tachometer, but I haven't worked GA maintenance in some time. He might have thought that it was a fluid-filled instrument, but I doubt that's the case. Most light aircraft use a mechanical tachometer. The camshaft turns a drive in the engine's accessory case. A sheathed cable connects to this drive, which spins the cable inside the sheath at camshaft RPM. The cable runs into the back of the tachometer and spins a cylindrical magnet which is nested within, but doesn't touch, a metal drag cup. The magnetic field induces a current in the drag cup and causes it to rotate (not all the way around) an amount proportional to the engine speed. That cup is connected to the needle through a mechanical linkage to give you an indication on the instrument face. It seems to me that the cold probably caused the linkage to develop a stiff spot, past which the small force produced by the drag cup was unable to overcome. There might even be some lubricant inside the instrument that gets thick in cold weather.

That's my guess, anyway. Could be way off.

 

[ppragman]

I've seen stuck tachs before, I've seen them spike, and do all sorts of weird things. Ummm, also, put it in the hangar overnight and let it warm up see if it does.

 

[unclenobby]

Thanks for the responses - the a/c was hangered and plugged in overnight but not plugged in when I got there (noon-time and hanger door was open). Plugged it in for the preflight so it got about 10 minutes heat (I did feel some heat in there). I asked for preheat but they were slow bringing it out so I tried to start it up and it did (after the 3rd try) so I just pressed on.

Since it was my first time experiencing this didn't want to take a chance even though through it felt like full power. Will just wait for preheat in future maybe thats the key.

 

[spilot]

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