N120GX

On September 9, 2020, at 0935 eastern daylight time, a Mooney M20R airplane, N120GX, was substantially damaged when it was involved in an accident near Weston, Florida. The pilot and passenger were not injured. The airplane was operated as a Title 14 Code of Federal Regulations Part 91 personal flight.

According to the pilot, the airplane had 6.5 quarts of oil (8 quarts total capacity) and 65 gallons of fuel onboard before departure for the 35-minute flight from Naples to Fort Lauderdale, Florida. About 20 minutes into the flight, at an altitude of 3,500 ft mean seal level, he “heard a loud pop and the prop sputtered and the engine started losing power.” He noticed that the oil pressure had decreased from 58-60 psi to 0 psi; however, all other engine gauges, including the oil temperature, were normal except for a “larger than normal draw on the battery.” About 30 seconds later, part of the engine exited through the top of the engine cowling, and the engine and propeller stopped completely. The pilot performed a successful forced landing to the right shoulder of an interstate highway where the airplane came to a stop. As the pilot and passenger prepared to disembark, a truck struck the airplane’s left wing from behind, and the airplane spun around 180°.

Examination of the airplane at the accident site by a Federal Aviation Administration inspector revealed that the airplane sustained substantial damage to the left aileron, the left wing trailing edge forward of the aileron, and the inboard trailing edges of both the left and right elevators. A hole was present on the top left side of the engine cowling and in the engine case near the base of the No. 6 cylinder. Metal debris, including a damaged connecting rod, its separated cap, a piston wristpin, a valve lifter, and crankcase fragments were found in the engine’s oil pan.

Examination of the engine revealed discoloration/thermal damage to one of the crankshaft main bearings, one of the crankshaft connecting rod journals, and two of the connecting rods (and bearings) at their crankshaft ends. The discoloration and damage were consistent with a lack of lubrication. The examination found no anomalies with the engine oil pump, and no blocked oil passages were found in the crankshaft.

The pilot/owner reported that on the previous day, he completed a long cross-country trip (four flights over four days, totaling about 16 hours). Over the course of the last two flights of that trip (which totaled about 7.25 flight hours), the engine used about 3.5 quarts of oil, or about 1 quart every 2.07 hours. When he added 2 quarts of oil after completing the trip, which brought the oil level to 6.5 quarts, he noted that the oil consumption “seemed to be on the high side.” Previously, he had added oil prior to two other legs of the trip, and the lowest oil level observed was “just below 6 quarts.” He recalled the recent oil additions from memory as he did not keep a written log of every addition. He did not report observing any oil leaks. The accident flight occurred the next day, which was a planned trip to get the oil changed. He intended to ask the maintainer to determine if the oil consumption was normal or to determine a reason for it, if not.

The airplane was equipped with electronic primary and multifunction displays, which recorded flight data to an on-board memory card. A review of the data from 34 previous flights revealed that during cruise flight, the engine oil pressure was typically between 60 and 65 psi, and consistent. Any fluctuations in cruise flight were normally concurrent with a change in engine rpm or oil temperature. Figure 1 shows the engine rpm, oil pressure and temperature for a typical flight.

Figure 1 - Engine Oil Pressure - Typical Flight

Beginning with the 3rd previous flight (which occurred 6 days prior to the accident), the recorded data for engine oil pressure deviated from the preceding typical trends. During the 3rd and 2nd previous flights, the oil pressure began to decrease about halfway through the flight, without an accompanying change in oil temperature or engine rpm. Additionally, during the decreasing trend, the oil pressure data became “noisy”, exhibited by high frequency fluctuations in the recorded pressure. These trends continued for the remainder of both these flights. The decrease in pressure was gradual, dropping 15-20 psi over about 90 minutes, reaching about 45 psi by the end of the cruise. A review of the data from 31 flights prior to this time revealed that the oil pressure was not below 60 psi during the cruise phase. Figure 2 shows the data from the 2nd previous flight.

Oil Pressure: Decreasing Trend and Fluctuations Begin

Figure 2 – Engine Oil Pressure - Data from 2nd Previous Flight Prior to Accident Flight

During the flight prior to the accident, the oil pressure did not reach the typical 60-65 psi during cruise, and it decreased throughout the entire flight. About 1/3 of the way through the flight, the pressure fell into the yellow range of the oil pressure gauge (10-30 psi). It continued to decrease until it briefly reached the lower red range (<10 psi) twice, near the end of the flight. Figure 3 shows the data from the previous flight.

Figure 3 Engine Oil Pressure - Data from Flight Prior to Accident Flight

During the accident flight, the oil pressure was generally more consistent with a typical flight, with a decrease (about 7-8 psi) over the 3 minutes prior to the loss of engine power. Figure 4 shows the data from the accident flight.

Figure 4 Engine Oil Pressure - Data from Accident Flight

Engine Instruments

Oil pressure and other engine indications are displayed on the airplane’s multifunction display (MFD). While this display is used for many different functions, engine data are typically depicted on the left side of the screen as shown in an exemplar image in figure 5. In some cases, such as when using the ‘lean’ function, the oil pressure gauge is not depicted. According to the MFD pilot’s guide:

“When unsafe operating conditions occur, the corresponding readouts flash to indicate cautions and warnings.”

The pilot’s guide did not specify whether the oil pressure indicator would flash if the oil pressure value fell within the yellow range.

The airplane was not equipped with a secondary oil pressure gauge.

Oil Pressure Gauge

Figure 5 - Exemplar MFD Display (Mooney M20M)

A review of the engine maintenance logbook revealed that the engine was manufactured May 2006, was installed when the airplane was manufactured, and had accrued a total of 1,166 hours. It had not been overhauled. According to the engine manufacturer’s overhaul manual, the recommended time between overhauls was 2,000 hours or 12 years, whichever comes first. The manual states in part:

“Regardless if the engine has been operated regularly or has been in storage; gaskets, seals, and synthetic and natural rubber goods deteriorate over time. Replace or overhaul the engine upon accumulating the operating hours specified in Table 6-1 [2,000 hours], or twelve (12) years after being placed in service, whichever occurs first.”

The most recent maintenance was an oil change, about 38 flight hours prior to the accident flight, on July 10, 2020. The oil filter was opened and examined at that time with no anomalies noted.

The most recent annual inspection was performed about 81 flight hours prior to the accident flight, on May 29, 2020, with no anomalies noted in the logbook entry. That logbook entry included a note that the oil filter was cut open and inspected, with no debris or metal detected. However, a separate supplemental document for that same inspection entitled “Mooney International Corporation 50-Hour/100-Hour/Annual Maintenance Inspection Guide” noted in one section “Dirty Oil – gunk in filter”. The remarks section at the end of the document read “oil was awfully dirty, filter was not clean, gunk like substance in filter folds (element).” The cylinder compression test values did not vary significantly between the previous annual inspection (May 2019) and the most recent annual inspection, except for cylinder number 2, which increased by 20 psi.

A review of the last 10 oil changes prior to the accident flight (including the two noted above) revealed oil change intervals that ranged from 21 to 46 hours. Of these, there were no other anomalies noted about the oil condition or mention of any debris found in the oil filter. However, the records for 5 of these oil changes did not specifically indicate if the oil filter was opened and examined.