Low RPM

orange

orange

Line Up and Wait
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Orange
Happy New Year all.

I need you to tell me if I did the right thing or if I was overcautious, could it be a explainable reason for what happened.

I rented a Cherokee 160 this morning, the same type that I usually fly. It was plugged in overnight, so it started on the first try. Taxi and run-up were both normal. Carb heat was off. Temps/pressures were good, I started my take off roll, slowly adding power to full. As I passed 40 knots (rotate at 60-65), I looked at my tachometer and it showed 2100-2200 RPM, the needle was jumping between the two. It's usually around 2600. I didn't even try to rotate, I just aborted the takeoff and went back to the school and parked it and went home. Maybe it was not that big a deal, but I'm always going to play it safe. If I feel anything is off, I'm always gonna pass. There's always tomorrow. It sucked to drive 35 miles each way to the airport but **** happens.

The school was closed, so nobody to talk to. I did leave a note for the next renter (I see 2 more today after me).
 
If the engine wasn't producing rated power, a takeoff abort was the correct answer.
 
Cherokee 160's airspeed is in MPH, not knots.

Did you have both mags "on?"

Most of these old airplanes have tachometers that read low.
 
Someone that flies it all the time can feel if it has full power on takeoff run. Do that test then decide.
 
coloradobluesky said:
Someone that flies it all the time can feel if it has full power on takeoff run. Do that test then decide.
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Anyone at all can access the TCDS database, where the static RPM range is given for various aircraft/engine/prop combinations. Lock the brakes, run the engine to full power and see what the tach says. If the reading is low, the tach is either way out of calibration and should have been replaced long ago or the engine is sick.

http://www.airweb.faa.gov/Regulatory_and_Guidance_Library/rgMakeModel.nsf/MainFrame?OpenFrameSet

Go to the "By Make" and select P and then Piper and then the model designation. There are a couple of PA-28-160s listed there; pick the one that has the serial number range for the airplane in question, and under propeller and propeller limits the Static RPM ranges are given.

In any case, the OP's 2100-2200 RPM is low; especially considering he had forward speed and wasn't static. He was right to abort.

In Canda we have a legal requirement to check the tach for accuracy every year. An error of 4% is the allowable limit, measured in the middle of the cruise RPM range. I looked through the FARs but can't find an equivalent regulation.
 
Bouncing tach needle can also be a gauge or cable issue. The engine could have been fine but the tach could be buggy. Never hurts to have a mechanic look at it before the next flight. Better safe than sorry!
 
I would have made the same call.

On a side note, would a low rpm like this be similar to how an airplane would perform at a higher density altitude? I've often wondered of high altitude operations could be simulated by just setting the engine to a lower power output.

In the little planes I fly, I always use full power for takeoff. Just curious.
 
ChrisK said:
2600 seems a bit high to me, but I'm not a mechanic...
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Keep in mind he was describing what he was used to seeing well into the takeoff roll.

You probably aren't going to see it that high with a fixed pitch prop at static rpm.
 
Somedudeintn said:
On a side note, would a low rpm like this be similar to how an airplane would perform at a higher density altitude?
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If you were at a high altitude airport and left the mixture full rich, you'd get similar results.
 
could be any number of things but one that is often overlooked is a chunk of the muffler baffle blocking the exhaust.
 
I have a piper 180 and I've never hit 2600 rpm on takeoff, ever. Granted I also takeoff at sea level with rich mixture so your mileage may vary.

I've found just about 2300 RPM is what I get with the throttle firewalled. At about 1000 feet that goes up to normal RPM and when cruising at around 6K I'm at about 2550 or so.

Never bothered me as long as I'm making speed. Though if I did see something much lower than that I would definitely abort.
 
To respond to some posts, I fly these only these Cherokees, so I'm pretty familiar with what it's supposed look, feel, sound like. The school has 5, I have flown them all at some point during training and after getting licensed. Each one is a little different, I haven't flown this one in a very a long time so I don't remember what quirks it has. On the take off roll, the engine sound was normal, but the power seemed a little sluggish, the speed was building, but I ALWAYS check the tach and it's high (over 2500), so when I saw 2100, up to 2200, that was NOT normal. That's why I immediately aborted. Like the saying goes, I'd rather be on the ground wishing to be up there, than be up there regretting taking off and hopoing to get back down on ground in one piece.

Ignition was on both, and carb heat was off. Those are 2 things that could have caused the lowwer RPM. After getting back to the school and before shutting it down, I set the parking brake, stepped on the brakes, and went full throttle, I got 2150 max. No good.

There's another day to fly.
 
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My first thought was carb ice as you taxied around. What was the outside temp/humidity? You did say it was plugged in, but that does not warm up the tach cable.

Run up checked normal, but did it feel like you had to use more throttle position to get to the rpm level for the run up?

I'll agree, if it does not feel right, don't go. Surprised the school does not have a number to call for maintenance issues.

Be interesting to know if anyone flew it after you decided not to. If I was the 2nd renter in line, I'd be upset for no calls from the school that the airplane was down and no substitute airplane available after a long drive to the airport.
 
Somedudeintn said:
I would have made the same call.



On a side note, would a low rpm like this be similar to how an airplane would perform at a higher density altitude? I've often wondered of high altitude operations could be simulated by just setting the engine to a lower power output.



In the little planes I fly, I always use full power for takeoff. Just curious.
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Kinda. Many people use reduced power to simulate high DA at lower airports.

It won't adequately simulate the higher ground speed, however. Which is one of the things that can trip up a pilot used to "things looking faster" who keeps pulling power on final and not watching the airspeed, or trying to lift off by how fast things "look" going by on takeoff.

Obviously the ground speed difference is relatively minor unless you're landing at LXV on a warm day.

I see it, but usually it gets me going the other way, since home is at 6000' MSL roughly. I glance outside when landing at or near sea level and have a tiny worry that things look way too slow and want to instinctively push the power up.

I'll also land flatter than usual if I don't let it keep slowing and holding it off. I tend toward three-pointers at sea level if not concentrating enough.

Exacerbated in my airplane by the STOL kit. It'll essentially still be flying right above a fast taxi speed at sea level. Fun. But utterly ridiculous in a 182. They already land plenty slow without it.
 
BillTIZ said:
My first thought was carb ice as you taxied around. What was the outside temp/humidity? You did say it was plugged in, but that does not warm up the tach cable.

Run up checked normal, but did it feel like you had to use more throttle position to get to the rpm level for the run up?

I'll agree, if it does not feel right, don't go. Surprised the school does not have a number to call for maintenance issues.

Be interesting to know if anyone flew it after you decided not to. If I was the 2nd renter in line, I'd be upset for no calls from the school that the airplane was down and no substitute airplane available after a long drive to the airport.
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Hey,
To answer your questions, it was 39 degrees F and 65% Humidity on that day.

The run-up was normal, I didn't have to use more throttle than normal.

I got in touch with the school later in the day and he told me that that particular Cherokee has intermittent tach issues, meaning issues with the gauge. Like I said, I had not flown that particular one in months. I scheduled for a different plane but they switched me over to this one. During training one time with my CFI, were climbing out after take off when at 900 feet or so, engine started sputtering, we turned back and landed safely on the other runway with Fire/Rescue truck standing at the side of the runway. LOL

I don't know if the other renters flew that day. I left a note explaining what I noticed.

I spoke to my CFI today and confirmed that he is looking for 2400-2500 RPM for takeoff at full power. He said that he would have done the same thing.

Thanks for replying
 
You are never wrong not to fly when you think something is wrong even if you are wrong about it being wrong.
 
Exactly, takeoffs are optional but landings are mandatory.

I've aborted many a takeoff for reasons similar to what you describe. Better to be safe then have something catastrophic happen in the air.

Regardless of the when/why/how and the armchair discussions of RPM and what it should be, you did the correct thing as PIC and made a call which given the circumstances ended up being the right one.
 
Probably an issue with the tachometer, but better safe than sorry!
 
That happened to me once in my old 235. It was normally around 2475 on takeoff. But on this flight it was like 2250. Slightly odd, but I had 8,000ft of runway so I tried to lengthen my rotate time...
Except the damn plane seemed to be making more power than I typically experienced as it rotated in about 6 seconds after releasing brakes.

Turned out to be a tach error on a cold day -_-
 
orange said:
I got in touch with the school later in the day and he told me that that particular Cherokee has intermittent tach issues, meaning issues with the gauge.
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Typical rental excuses and garbage maintenance. Beware.
 
I've scrubbed flights for low tires. Good on the OP. What is it they say about take-offs being optional?
 
Yeah I would have aborted also. It might be a tach problem but it might not. Guess which side I'm gonna err on.

If the tach were really off by that much (2600 vs 2100?!) it should be fixed. If it's off by 100 or so, and not jumping about, it should be placarded.

Mine is 100 RPM low and placarded as such.
 
I've been thinking a digital tach would fix issues like this nicely. Why would anyone install a mechanical tach these days?
 
denverpilot said:
You think a digital tach can't fail or read erroneously? Seriously?
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It has fewer failure modes and it's less likely to be slightly off. If it fails, chances are it will be obvious. I'd prefer that over the guessing games with an analog tach.
 
denverpilot said:
You think a digital tach can't fail or read erroneously? Seriously?
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Not sure how you drew that from his post, but he didn't say that at all. He said less likely. And I agree with him. Mechanical tachs are notorious for errors.
 
fiveoboy01 said:
Not sure how you drew that from his post, but he didn't say that at all. He said less likely. And I agree with him. Mechanical tachs are notorious for errors.
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He didn't say "less likely" at all until his *second* post, which was after mine. But I'm sure he can explain these things himself... and did.
 
If you would have comprehended his FIRST post you'd also see that he didn't say it "couldn't fail" there either.

Next time I'll get the other guy's permission to post, sorry:lol:
 
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