Does this Tach seem off to you?

[drotto]

I did a solo practice area flight yesterday in a Warrior. The run-up went fine, the plane performed as expected. Initially I did 3 trips around the pattern, and all seemed fine, so I proceeded out to the practice area. I went to throttle back and the throttle was at the 1/2 mark before the RPM's on the Tach went below 2500. When I got the RPM's to read a typical 2200 to 2300 for cruise, the plane just seemed slower than normal, so I looked down and the airspeed was down around 90 knots at level flight. So I throttled up to get the airspeed back up to the cruise speed I typically see around 110 knots. To maintain that airspeed the tach was reading over 2500.

I did a few turns and some slow flight. The did a little more pattern work. While taxiing, the plane was close to idol, and when I looked at the Tach it was bouncing between 600 to 900. I went up to do the pattern one last time, and ended up doing a go around, because when I set the RPM to my typical descent speed, I was way to high and felt no need to force the landing. I went around this time ignored the tach, and had a solid landing. I reported the Tach to the machanic, and went home.

At no time did the engine not repond normal or miss a beat, the plane performed normally. At no time was the RMP reading in the red. Am I right thinking the Tach seemed off?

 

[MAKG1]

If it's moving around a lot and the engine sound isn't changing, it has a problem. Especially near idle.

Red tag the airplane and squawk it. It's not airworthy per 14 CFR 91.205.

 

[drotto]

If it's moving around a lot and the engine sound isn't changing, it has a problem. Especially near idle.

Red tag the airplane and squawk it. It's not airworthy per 14 CFR 91.205.

I squawked it, told a mechanic and the girl that rents the planes. The problem did not show on initial run-up, I only noticed it near idol on my last taxi.

I also know that plane has a history of tach failure, since it went bad about 5 months ago went I was up with a CFI. I had it marked in my logbook. I have not been in that plane much during my recent training, but my prefered plane was down for its 100 hr. I just will not fly that one again, just do not like it as much.

 

[timwinters]

see if someone around your airport has one of these that you can borrow.

 

[poadeleted20]

If you have a smartphone, you don't need a TruTach. There are acoustic tach apps for both Android and iPhone. The one I use on my Android phone is called "Head Speed Tachometer". It costs about $3 IIRC, and it works great. If I have any doubt about a tach in flight, out comes my phone and I check it.

 

[dmspilot]

You could try a full throttle static runup and see if the RPM matches the figure specified in the book.

 

[MAKG1]

You could try a full throttle static runup and see if the RPM matches the figure specified in the book.

That makes you vulnerable to canceling errors.

If the tach is reading 200 RPM fast, but the engine is actually mistimed and is running a static RPM 200 RPM low, your takeoff is going to be "interesting."

It's a mistake to treat a known-incorrect instrument as registering any reading at all.

 

[dmspilot]

If the tach is reading 200 RPM fast, but the engine is actually mistimed and is running a static RPM 200 RPM low, your takeoff is going to be "interesting."

Well, if it matches, the result is inconclusive. If it doesn't match, there is conclusively something wrong.

It's a mistake to treat a known-incorrect instrument as registering any reading at all.

But he doesn't know if it's incorrect or not. Isn't that the whole point.

 

[MAKG1]

If you have a smartphone, you don't need a TruTach. There are acoustic tach apps for both Android and iPhone. The one I use on my Android phone is called "Head Speed Tachometer". It costs about $3 IIRC, and it works great. If I have any doubt about a tach in flight, out comes my phone and I check it.

You're kidding, right?

The aircraft is unairworthy. The OP diagnosed the problem just as he should have -- by being familiar with the aircraft performance numbers -- and found a fabulous way to get it safely on the ground, and it should stay there.

What's next, an iPad app to warn the gear is up? How about a "this is stupid" app that tells you conditions exceed your skills? Sometimes, we have to use our brains instead of the latest iToy.

"Works great" is not an evaluation. Every measurement tool has limits. What are they? Will it work in a twin? How about with radio static? How about on a geared engine (much higher RPM)? Sometimes engines have notes at half RPM in the presence of valve train problems. Spark noise is almost always at half RPM. The exhaust note is at half RPM, but the prop is at full RPM (excepting geared engines); which is the louder note is a function of engine settings. Engine accessories and forced air induction will make incommensurate frequencies. Digital tachs are also vulnerable to sampling noise and aliasing.

The optical tachs have fewer variables to sort through, though some of the above failure modes apply. There really is only one moving part up front that you can see (or maybe two on opposite sides of the aircraft). But the airplane is on the ground and is not airworthy, so aside from keeping one in your flight bag all the time, there is no use for this device.

I believe the OP is a student pilot. Not for long, I hope. That's some serious PIC work.

 

[poadeleted20]

You're kidding, right?

No, I'm not kidding. The smartphone apps are an excellent way to confirm one's suspicion that the tach is inaccurate. It helps determine whether the problem is the tach or the engine or some other system so you can take appropriate action.

 

[timwinters]

If you have a smartphone, you don't need a TruTach. There are acoustic tach apps for both Android and iPhone. The one I use on my Android phone is called "Head Speed Tachometer". It costs about $3 IIRC, and it works great. If I have any doubt about a tach in flight, out comes my phone and I check it.

That's interesting, I didn't know that...but should have assumed that "there's an ap for that!"

I bought my "prop tach" before the smart phone era. It's been a great resource in the cockpit.

 

[drotto]

The aircraft is unairworthy. The OP diagnosed the problem just as he should have -- by being familiar with the aircraft performance numbers -- and found a fabulous way to get it safely on the ground, and it should stay there.

I believe the OP is a student pilot. Not for long, I hope. That's some serious PIC work.

Yes, I am a student. Thanks for the compliments, but probably not needed. When one thing was not working right, I used the stuff which was working just fine, ie airspeed and descent rate. Never did the plane's performance worry me, just needed to ajust my thinking for landing.

I wil likely be a student pilot for a bit longer. I am at 29 hours, and this was my first away from airport solo. Still need to do nights, xc, solo xc, and more general practice.

 

[MAKG1]

Good. I thought you were a bit further along than that.

But it does show that s*** happens during student solo flight. Much more than one might expect ahead of time.

The partial panel skill is a good one. Don't sell yourself short.

I had no less than four unusual events occur in my long solo cross country. One was an emergency landing (not mine) directly below me, as Approach lost touch due to terrain (the landing was fine).

 

[coloradobluesky]

You can check a tachometer accuracy under a fluorescent light. Two blade prop synchs at 1200 and 2400 rpm as I recall (and I could be wrong, its been a while since I figured this out). It has to do with the fact that the light blinks in synch with the 60 cycle electrical power grid. Do it with the light at your back. The prop will appear to stop at the the synch points.

 

[Captain]

Oy,

Okay, you found the plane good enough to proceed out to the practice area, come back, do a few circuits in th pattern, land, taxi back, take off again, and do another circuit or two...BUT NOT good enough for the next renter?

My point isn't that you shouldn't have written it up but rather that you continued voluntary flight way past when you found an instrument not working right. I know, performance was there and you didn't have other symptoms...but I submit you're not in a position to know being a student with 25 hours. If it's not exactly like it should be then stop and sort it out with your instructor, another instructor, or someone who isn't still doing primary training.

 

[Henning]

I did a solo practice area flight yesterday in a Warrior. The run-up went fine, the plane performed as expected. Initially I did 3 trips around the pattern, and all seemed fine, so I proceeded out to the practice area. I went to throttle back and the throttle was at the 1/2 mark before the RPM's on the Tach went below 2500. When I got the RPM's to read a typical 2200 to 2300 for cruise, the plane just seemed slower than normal, so I looked down and the airspeed was down around 90 knots at level flight. So I throttled up to get the airspeed back up to the cruise speed I typically see around 110 knots. To maintain that airspeed the tach was reading over 2500.

I did a few turns and some slow flight. The did a little more pattern work. While taxiing, the plane was close to idol, and when I looked at the Tach it was bouncing between 600 to 900. I went up to do the pattern one last time, and ended up doing a go around, because when I set the RPM to my typical descent speed, I was way to high and felt no need to force the landing. I went around this time ignored the tach, and had a solid landing. I reported the Tach to the machanic, and went home.

At no time did the engine not repond normal or miss a beat, the plane performed normally. At no time was the RMP reading in the red. Am I right thinking the Tach seemed off?

Sounds like a tach problem, not uncommon with mechanical, magnetic couple, spring bound tachs, nothing to get particularly fussed about. You shouldn't really be using it too much anyway. As long as the engine sounds right, it is right. Use your trim to set your airspeed and your throttle to control your climb and descent. Learn to control the rough setting of the throttle with your ear and fine tune with your rate of sink as judged by the upward and downward shift in the perspective lines coming to meet you from the ground. If your aim point moves up in the windscreen and you appear to be sinking in relation to the perspective lines, add throttle. If your aim point is sinking, reduce throttle, slow down, and or slip, as necessary.