Seneca - weird manifold pressure gauge indications/behavior
Posted by ltcterry@reddit | flying | View on Reddit | 7 comments
A friend bought a Seneca and wants me to prepare him for initial Commercial in it. We did a lesson yesterday, our first actual lesson other than ferrying the airplane. Once we leveled off in cruise, the manifold pressure gauge began to act strangely.
Imagine you are in cruise: prop, throttle, and mixture are set. MP is at 30 inches. On a single two-needle gauge. Then slowly one needle moves towards 20" while the other moves towards 40". There is no change in sound. No yawing motion. So, no indication anything w/ the engines has actually changed. The split looks symmetrical.
The needle creeps up to about 42" and the overboost light comes on. A small movement of the throttle results in a huge movement of the needle. So, from an actual 30" to displaying 40-42", with the reduction resulting a likely accurate 25-27" showing. Retarding both throttles and going back to 30" resulted in this every time.
As we're trying to understand this, we've abandoned our plan for the 2-hour, 100-mile dual XC and are flying back to our home airport. It's not an emergency, but I don't want to land elsewhere.
About this time, I realize there's also a similar spread in the fuel flows. Any oddly huge difference.
Below 25 inches or so the needles seemed to make total sense.
How are the left and right manifold pressure systems interconnected? Seems like the only common point would be the gauge, but I really don't know. How is the fuel flow gauge connected to this?
Av8torryan@reddit
The manifold pressure indicator is nothing more than a vacuum gauge. I would likely suspect the guage is going bad if both engines are running the same fuel flow amount, and props are synced and prop levers are essentially matched . If one engine is running a much MP, than one blade is going to be a higher pitch than other and vice versa to match rpm’s. You’ll definitely get a very noticeable “wah wah wah “ sound
x4457@reddit
What did the mechanic you asked about this say?
ltcterry@reddit (OP)
He said “I don’t know much about Senecas. Push the manifold pressure line drain valves again and go fly.” Which I will try once the weather improves.
InGeorgeWeTrust_@reddit
You need a better mechanic lol with Seneca experience
ltcterry@reddit (OP)
Not my airplane. Not my mechanic. And the plane’s 130 miles from its eventual home base. But agree an A&P w/ Seneca experience is better than being someone’s training aid!
My guess is there’s an issue in the gauge w/ either a leak or something is flexing, hence the opposite direction symmetry of the indications.
BrtFrkwr@reddit
Sounds like a bad instrument. Avionics shop can put a vacuum and pressure on it and check it.
rFlyingTower@reddit
This is a copy of the original post body for posterity:
A friend bought a Seneca and wants me to prepare him for initial Commercial in it. We did a lesson yesterday, our first actual lesson other than ferrying the airplane. Once we leveled off in cruise, the manifold pressure gauge began to act strangely.
Imagine you are in cruise: prop, throttle, and mixture are set. MP is at 30 inches. On a single two-needle gauge. Then slowly one needle moves towards 20" while the other moves towards 40". There is no change in sound. No yawing motion. So, no indication anything w/ the engines has actually changed. The split looks symmetrical.
The needle creeps up to about 42" and the overboost light comes on. A small movement of the throttle results in a huge movement of the needle. So, from an actual 30" to displaying 40-42", with the reduction resulting a likely accurate 25-27" showing. Retarding both throttles and going back to 30" resulted in this every time.
As we're trying to understand this, we've abandoned our plan for the 2-hour, 100-mile dual XC and are flying back to our home airport. It's not an emergency, but I don't want to land elsewhere.
About this time, I realize there's also a similar spread in the fuel flows. Any oddly huge difference.
Below 25 inches or so the needles seemed to make total sense.
How are the left and right manifold pressure systems interconnected? Seems like the only common point would be the gauge, but I really don't know. How is the fuel flow gauge connected to this?
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