I’ve had two experiences with inflight loss of manifold pressure in my IVP. It can be an emergency depending of the situation. This is not intended to be training or advice about how to handle these situations, but I think we all benefit from sharing experiences. I’ll share with you my experiences in hopes it may be helpful to other pilots. I probably did not do everything right, but regular training helped me to recognize what was going on and how to deal with the situations.
My First Experience
It was a relatively clear spring day. I was at 16,000 feet over Eastern Iowa headed toward Minneapolis in VMC. The flight was uneventful, until it wasn’t. Without warning, I heard a loud bang, followed by an immediate loss of engine power and cabin pressure. Manifold pressure dropped from 30 inches to 12 inches and the cabin altitude pressure warning started to blare. My initial startle response led to the alarming thought that my engine had blown apart.
I quickly realized, however, the plane was flying and stable. Looking outside, I could not see any change in the plane’s appearance, and no signs of damage or leaking fluids. Scanning the engine gauges, everything seemed normal except the manifold pressure and the resulting loss of cabin pressure.
Cabin pressure was a problem, but not an emergency. I knew I would be able to function, although hypoxia would be a concern if cabin pressure was not resolved soon. I carry oxygen with me with the bottle readily accessible, but I chose not to immediately grab it. I was more concerned with the loss of manifold pressure. The autopilot was on and my airspeed was beginning to slow. I realized I could not hold altitude and would need to descend. In hindsight I suppose my decision not to immediately use supplemental oxygen could have worsened the situation—hypoxia is never helpful—but my thinking was that descending would make the issue moot.
Once I analyzed the situation I quickly narrowed the problem to the turbocharger. I was on an IFR flight plan, so I contacted ATC to tell them I had lost manifold pressure and was unable to hold altitude. Although "loss of manifold pressure" is likely superfluous information for most air traffic controllers, my inability to hold altitude was clearly a problem. ATC asked if I needed to declare an emergency. All our training says don’t be afraid to declare an emergency, but we all hesitate to do so because of the perceived hassle it will cause. I quickly debated this in my head before replying “no,” but requested 11,000 feet and a diversion to a Class C airport within gliding distance. I was pretty sure I could make the airport, but wanted to maintain as much altitude as possible while knowing I had to descend.
It didn't immediately dawn on me, but I eventually realized since I was now flying a normally aspirated engine the manifold pressure would recover as I descended—assuming there was nothing else wrong. That is exactly what happened; as I descended the manifold pressure recovered. I requested 8,000 feet and a clearance to another airport where I knew a mechanic who could help. Rather than landing at the Class C airport within gliding distance I chose to fly another 20 minutes to the mechanic. This seemed reasonable since the plane was flying fine. The landing was uneventful. When we uncowled the plane, we discovered one of the main hoses to a turbocharger had simply blown off and the fix was easy.
They say experience is the best teacher. I’m glad for my first experience with loss of manifold pressure experience as it prepared me for a much scarier situation. I was flying from Idaho to Nebraska in late winter. The weather was mostly clear at both the departure and destination airports, but was solid IMC from 400-600 feet all the way up to FL 230 almost the entire route between. I departed and climbed to FL 250 through roughly 8000 feet of IMC until reaching clear skies on top. I encountered no icing.
About an hour after departing, roughly over Jackson Hole, WY, I heard a bang, but not nearly as loud as in my first experience. My first thought was I hit a bird, but this seemed unlikely at that altitude. My next thought was a pressurization leak. Sure enough, cabin altitude immediately rose from roughly 8000 to over 20,000 feet. The plane didn’t seem to be falling out the sky, so my next thought was oxygen; time of useful consciousness can be as low as three minutes at that altitude without oxygen.
I don’t think I noticed a reduction in manifold pressure yet since I believed the problem was a pressurization leak. My immediate focus was on staying conscience. I donned the oxygen mask attached to the tank behind the co-pilot seat and turned on the oxygen. Then I assessed the situation and noticed manifold pressure was about 22 inches, significantly higher than in my first experience. It was very cold outside—minus 40 degrees centigrade—so I had the cabin heat and defrost on full. I turned them both off and slowly regained lost cabin pressure, bringing cabin altitude down to about 13,000 feet. Then, I made a dumb decision: Relieved that I could survive at 13,000 feet cabin pressure, I turned off the supplemental oxygen and set the mask aside.
The plane was flying fine. I seemed to be able to maintain altitude, but speed was very slowing decreasing. I was not comfortable flying for 2 1/2 hours to my destination with this condition, but I knew it was solid IMC below me all the way to the ground over high mountains with VMC conditions unlikely until close to my destination. The closest airport with services was Casper, WY about 100 miles away. I elected to divert and informed ATC. Again, they asked, and again I declined to declare an emergency. The plane was flying fine, although I was unsure if I’d be able to maintain altitude. ATC gave me a series of block altitudes, first from 24,000 feet to 18,000 feet and later 18,000 to 15,000 feet. My plan was to stay above the IMC as long as possible.
The plane continued to slow and at 150 knots I started a slow descent to an altitude just above the clouds. My initial climb gave me some confidence I wouldn’t encounter (at least severe) icing in the clouds but still, I didn’t want to linger. Plus, of course, I wanted to be well clear of the mountains. So, when I reached a distance from Casper where a 2000 feet per minute descent would get me straight into Casper, I pushed the nose over into IMC and started down. The descent was smooth and uneventful, but once I was in the clouds with no sunshine I realized the error of turning off the heat and defrost. My feet quickly became very cool and the windscreen iced over on the inside. This was resolved with a towel and some heat once I was below 10,000 feet.
I shot an IFR approach into Casper with a 400 feet ceiling and landed uneventfully. Removing the cowling revealed one of the smaller exhaust hoses became detached from the intercooler. This is why the manifold pressure drop was much less than in my first experience. Again, the fix was easy, and I enjoyed a day and a half getting to know Casper waiting for the weather to clear!
Several lessons were reinforced to me by these experiences. The first is to study your aircraft systems. You should know what can cause a loss of manifold pressure in your plane, what other systems are affected, and what you will do to address the problem in-flight under different flight scenarios.
The second is to properly maintain your aircraft. Cabin pressure and turbocharger hoses operate under significant pressure, and the soft silicone tends to “flow” and therefore loosen over time under the pressure of the clamps. The tightness of all these clamps needs to be checked at least at every condition inspection.
The third is to self critique following every flight, especially following unusual circumstances. What I learned about how I handled these circumstances is:
- Cross-check the consequences of my actions to make sure I'm getting the expected results.
- Carry supplemental oxygen.
- Don’t panic.
- Assess the situation to determine the most critical elements of the problem.
- Let ATC know what is going on and don't be afraid to ask for help.
- Return to service test flights are critical after any significant maintenance, but be aware some issues may not surface immediately, or even on the first flight following maintenance.
- And finally, carefully evaluate whether flying over mountains in IMC is advisable.
For questions and comments regarding the post contact Steven via email at s.bradford [at] lancairowners.com.