This is an excerpt from a report made to the Aviation Safety Reporting System. The narrative is written by the pilot, rather than FAA or NTSB officials. To maintain anonymity, many details, such as aircraft model or airport, are often scrubbed from the reports.
I was flying the second leg of a four-leg flight. The flight was generally uneventful VFR. The intention was to visually follow the highway as it departed ZZZ eastbound, as this was a route I was familiar with, and terrain allowed a comfortable crossing at 11,500 feet. Upon approach to the ZZZ area, there was a layer of scattered clouds at approximately 9,000 feet MSL, which complicated the visual acuity of the pass, and made it a marginal proposition. The decision process was to proceed south at 11,500 feet, which would add approximately 20 minutes to the flight or to climb.
Ultimately, oxygen was used and a climb initiated to 13,500 feet. During the climb, and due to the Baron’s normally aspirated engines, it was decided to change the propeller setting from 2400 rpm to 2700 rpm to provide additional climb rate.
The first sign of abnormality was during this climb, as the left propeller lagged the right in responding to the RPM change. The right immediately governed to 2700, while the left took approximately 10 seconds to match. It was a slow and steady rise in RPM, so it was not immediately obvious if this was an instrumentation error, propeller governor issue, or something else. Oil pressure and temp were normal, and the engine temperature instruments all seemed normal.
While crossing the area at 13,500 feet, I happened to notice white smoke streaming from the area behind the left propeller spinner. I checked oil pressure and it was normal. The engine instrumentation was normal with no warnings from the engine monitor instruments.
After a brief moment of reflection, I decided on a precautionary shutdown of the engine and a return to ZZZ airport, which I had just crossed over. The engine secured and feathered with no issue. The smoke abated after approximately 30 seconds and an oil streak was observed on top of the cowling. Additionally a rumbling vibration, not unlike a fouled spark plug, was noted.
I requested priority handling with ZZZ Center and turned to the airport. An ordinary landing was achieved without incident. Taxiing was slow and difficult, but I was called on UNICOM by the FBO and offered a tug. I shut down the aircraft and awaited the tug.
I was able to inspect the left engine by opening the cowling, and discovered a large hole in the engine crankcase at the #6 cylinder position. The connecting rod was discovered lying further back on the engine crankcase, astride cylinders 2 and 4. A piece of the crankcase that contained the 12 o’clock cylinder base stud boss was lying on top of the rocker covers between cylinder 2 and 4. The sump was checked and eight quarts of oil was verified, down approximately two quarts from our ZZZ1 departure.
No injuries were sustained and the majority of damage is mechanical in nature to the engine itself.
Upon reflection, I think a few things are worth noting with this incident.
First, pilots are often hesitant to request priority handling for marginal or “assured” situations. Our situation was never in doubt, but I requested early. This paid dividends as I later learned that the FBO was contacted by ZZZ Center to confirm my landing — the FBO staff did not note my radio calls. This allowed ground support to dispatch — something I would have never considered as a benefit of requesting.
I did not need to use my 91.3(b) authority, however, I wanted the added attention and service in the event that the aircraft’s mechanical condition deteriorated further.
Second, it is rather unsettling that the left engine on the aircraft was actively destroying itself and the resulting vibration was mostly imperceptible. This is a testament to the engine design, but also a concern in multi-engine aircraft I had not previously considered.
I assumed that “thrown rod” incidents would always include a loud bang and a cacophony of noise and vibration. This was calm, and given the mountain turbulence already surrounding us, barely perceptible. The power loss was not even noticeable due to the high altitude and our operating near 40% power.
Had the flight been at night or under IFR, the smoke would not have been detected, and the engine would likely have seized before I took action, possibly preventing procedures from succeeding and making the situation more dangerous.
Third, I have had minor mechanical mishaps in the past at towered airports, and this resulted in a lengthy interview that followed with the area FSDO. I understand that this is a recent policy that all mechanical incidents must be investigated in depth.
Because so much administrative time was wasted on prior trivialities (for example, a precautionary aborted takeoff due to an alternator going offline during the takeoff roll) I very strongly hesitated in requesting priority handling as the situation was benign and manageable.
I think the policy of mandatory reports from towers/agencies for mechanical aborts creates a chilling effect and is actually detrimental to safety, as it may cause a pilot to continue in a marginal mechanical situation “for fear of the interview with the FAA inspectors.”
The inspectors I have spoken with in the past also seem perturbed by what amounts to busy-work and investigations of pilots who are acting proactively and with an abundance of caution.
Primary Problem: Aircraft