Hi everyone,

 

Recently, my 1967 PA30 with Lycoming IO-320-B came out of the shop after some mx due to right rough engine and backfiring.

Before going into the shop, I used to cruise close to 160KIAS at 22" and 2300RPM which is pretty normal. The aircraft has Knots2U modifications.

After it came out of the shop, now I can hardly maintain a climb at 90, and at above power setting its max cruise now is only 105KIAS. Nothing hanging out and all clean, cowl flaps closed etc.! Engine at TO all checks normal, and climb power is at 25" 2500RPM. During the original problem repairs, fuel servo, plugs was changed. Cracked exhaust system was also changed.

What happened to the airspeed I used to have????? At climb power, I used to be able to get 1500fpm at 115IAS. Now, I can't even get it to 115IAS

Engines, fuel servo, pumps all were checked out and within limits. Props seem to be normal. The aircraft does seem a bit sluggish during climb and cruise. Numerous maintenance engineers are at a loss as to what's causing the slow speed. And its not just 10-15kts but at least 50kts!

I would really appreciate any thoughts and suggestions that you can offer. We are at wits end.

Thank you,
Robert

That sounds really dangerous. As in abort takeoff dangerous. Are you getting full rpm from the engines on takeoff?

 

You confirmed that the flaps and gear are up. The airplane is clean. So it is a low power problem.

What would cause BOTH engines to lose the exact same amount of power so as to avoid a huge amount of asymmetric thrust?

I have, a few times of the years, forgotten to raise my flaps after takeoff and have attempted to trim out with the flaps down in various aircraft. I usually figure it out during cruise checks as the speed is way to low and the noise is way too loud.

Mysterious matched low power: what if the tach was indicating high (which is weird for the tach) and they set the governers to an actual lower rpm while indicating a higher rpm. That might rob you of the top 200 or 300 rpm on both engines? Double check your tach against digital tachs?

I am at a bit of a loss to guess at what might rob both engines of power to the same degree other than fuel supply. Perhaps a batch of bad gas? Something in the fuel? Perhaps drain the tanks and try with fresh fuel?

I would check the pitot-static system first.

Are you positive the airspeed indicator is indicating the correct airspeed?

 

 

 

Interesting. Mag timings? Could they have installed all four mags incorrectly and to exactly the same degree? Perhaps some parts of the ignition are hooked up incorrectly and you have sparks happening in the wrong places?

 

I have to say, since what you are describing requires both engines to be messed up to the same degree, I can't but come back to the idea that there is something hanging out in the breeze. When you describe the way it flies, perhaps something is not coming all the way up, or the ailerons are very badly rigged, or the flaps are actually down when you think they are up, or the trim is odd somehow?

Can you/did you check the fuel burn vs the rated power it should be making?

How sure are you the gear is all the way up? I mean, the light could be on and the lever mostly on the floor, but there could still be a lot of tire hanging out, or perhaps the doors are open?

 

I'm a new owner, so no expert, but perhaps you have the alternate air selected "on" on both engines? That would take some HP out of the equation.

 

Still, not really sure how you got airborne... Even with the gear down, my plane wants to keep accelerating on takeoff...sounds like you're not even getting to blue line.

Good luck!

 

I would be looking at what was tinkered with. You mentioned the fuel servo's were touched. What's your fuel flow during TO? Has anyone checked the fuel flow at each nozzle? I would also be pulling the spark plugs that were just replaced. Have them properly checked and not just with a VOM.

 

What's your fuel flow at 22" 2300rpm?
Is she running rough and if so at what power settings. Wondering if you have partially plugged injectors causing more fuel to the other cylinders. That would require both engines to have the same injector issues. Consider your fuel source. Did you get fuel from a credible source. Has anyone taken a sample and had it analysed.

I'm no A&P but these engines are basic.

Have you found the problem yet?

 

My first guess was a pitot static problem, but you seem to have ruled that out with the GPS readout.

From what you are saying I don't think you have a rigging or airframe problem.

Was the same work done on both engines? By that I mean fuel servo work, plugs etc.

It's hard to believe the same fault could be introduced on both engines at the same time, but I guess anything is possible.

I presume the engines are running smoothly, this to my mind would rule out plugged injectors.

Have you checked the MP reading withe engines stopped, do they read about 29 - 30 inches. What is the reading at full throttle, it should be very close to what you see when the engines are stopped.

What RPM are you showing at full throttle with the props in fine? 2700 RPM?

What fuel flow are you seeing at Take Off power? About 12 USG/hr plus or minus 1 or so GPH? The fuel flow will, give you a good idea on the amount of power you are getting. If the flow is down then power will be down.

Going back to basics, the aircraft performed OK prior to the work so it's most likely your problem results from that work. What was done that might cause a problem to both engines. I'd be suspicious of the work done on the Fuel Servos.

I forgot to add above that if an engine with a constant speed prop fitted is down on power it can be hard to detect initially.

 

An engine with low power output can show normal cruise power settings, i.e. 23" 2400 RPM. However these readings do not show torque or actual power output as the constant speed unit keeps the RPM correct by changing the prop blade angle to keep the set RPM, so even with a low power output you can see normal power settings on the guages.

One other cause of your problem could be leaking manifold pressure lines (why two would become faulty at the same time I don't know) allowing atmospheric pressure into the line thus giving a false high reading, meaning for a normal MP indication you will set a lower throttle setting. At Take off power on a normally aspirated engine everything will indicate correctly as the MP should be the same as atmospheric so the leak has no effect. Once you start reducing power the leak will cause the gauge to read higher than normal.

What MP do you see at idle? It should be quite low somewhere around 12" from memory. If its much higher than this then you may have a leak in the MP line.

You don't mention any issues on take off. Does the aircraft accelerate normally? Do you have 29 - 30" MP, 2700 RPM and 12 GPH plus or minus on take off? If it accelerates normally and the MP, RPM and FF figures are normal then I'd say the engines are performing OK but you likely have a leak in the MP lines.

How far do you need to pull the throttles back to reduce to climb power? If it's a long way further than it used to be I'd say that's another indication of a MP line leak.

Roger,
I would suggest a flight in good VMC with an experienced (owner) Comanche pilot beside you. The numbers you describe are very, very, odd. Before and after maintenance.
Cheers,
Trevor