N9574XCessna 210B

On May 30, 2023, about 1115 Pacific daylight time, a Cessna 210B, N9574X, was substantially damaged when it was involved in an accident near Tacoma, Washington. The flight instructor and student pilotwere not injured. The airplane was operated as a Title 14 Code of Federal Regulations Part 91 instructional flight.

The flight instructor stated that she intended to perform a practice landing at Boeing Field in Seattle, Washington. While on the downwind leg of the traffic pattern, she completed the landing checklist but encountered an issue with the landing gear. The landing gear did not fully extend and appeared to be stuck in a transitional position. The flight instructor attempted to resolve the situation by moving the gear selector back to the gear-up position but observed no landing gear movement. The gear selector was then returned to the gear-down position, but again no landing gear movement occurred. Suspecting a problem in the hydraulic system, the flight instructor attempted to extend the hydraulically actuated wing-flaps, which was unsuccessful.

The flight instructor requested to perform a low-approach over the runway so the air traffic controllers could visually verify the landing gear position. The controllers stated that the landing gear appeared to be stuck in transition, and the flight instructor opted to divert to a practice area to troubleshoot the landing gear. She used the emergency backup hand-pump, but her attempts were unsuccessful. She called a maintenance technician, and he attempted to troubleshoot the landing gear, but the situation did not change. The flight instructor then diverted to Tacoma Narrows Airport, Tacoma, Washington and performed several low passes over mechanics on the ground who assessed the condition of the airplane’s landing gear.

After about 1.5 hours of troubleshooting, the flight instructor decided to land with the gear partially extended. The airplane touched down on the centerline of the runway with the nose gear locked in place, which allowed for limited turning ability. The airplane came to rest in the grass off the left side of the runway. The left horizontal stabilizer was damaged during the accident sequence (see Figure 1).

Figure 1: Airplane damaged at the accident site.

A comparison of the Airworthiness Directive (AD) compliance listing in the logbook against a list of AD's applicable to the airplane’s serial number revealed that all those pertinent had been endorsed as complied with. This included 76-04-01, which was shown as complied with in August 1985.

A comparison of the Airworthiness Directive (AD) compliance listing in the logbook against a list of AD's applicable to the airplane’s serial number revealed that all those pertinent had been endorsed as complied with. This included 76-04-01, which was shown as complied with in August 1985.

Actuator

Cessna sourced actuators from two companies: Electrol Inc. and Ozone. The Electrol Inc. unit used a snap ring to hold the end plug in place, with a cover plate positioned over the end plug to pull it out to the snap ring and secure it. The Ozone design did not use a snap ring. Instead, the end cap was screwed in, which was a more robust design. A Service Bulletin was issued to address inspections of these actuators.

Prior Accidents

This is at least the fourth known failure of this actuator, and numerous other accidents may have been a result of other actuator failure, but those units could not be examined. The National Transportation Safety Board has examined and investigated the actuators of 4 accidents (including this one):

Cessna 210 accident that occurred in Juneau, Alaska in 2015

-Actuator P/N EA1614, S/N 267 (Electrol Inc.)

Cessna 210C accident that occurred in Juneau, Alaska in 2018

-Actuator P/N EA1614…, S/N 275… (Most of placard obliterated)

Cessna 210B accident that occurred in Clinton, Arkansas in 2022

-Actuator P/N EA161402, S/N 190 (Cox Airparts)

The fractures from all these accidents were identical. The location, size, and physical features of the cracks were consistent and included fatigue cracks initiating from the retainer clip grooves on the interior of the actuator housings that propagated outward into the housing bodies. When the crack had grown large enough, the housing fractured, relieving hydraulic pressure, and causing the actuator to fail in service.

Maintenance Inspections

There have been numerous service bulletins and airworthiness directives issued on these types of hydraulic actuators over the last several decades. These include:

Cessna Service Letter 67-16 (March 28, 1967) on replacement

-Covering Electrol actuators 1813-1/-2 with S/N 1-31 (except 27) and 1-32 (except 28-29), respectively

-Covering Electrol actuators 1614-1/-2 with S/N 405-415, 448-468 and 400-404, 444-453, respectively

Cessna Service Letter SE69-17 (September 16, 1969) on field repair

-Covering all actuators installed on 1964 Cessna 210s

Cessna Service Letter SE75-21 (October 3, 1975) on modification

-Covering Electrol actuators installed 1960 to 1964 in Cessna 210s

Airworthiness Directive 76-04-01 (January 27, 1977) on repair/replacement

-Covering Cessna P/N 1280102-1/-2 or Electrol 1471-1/-2

-Covering Cessna P/N 1280501-1/-2 or Electrol 1614-1/-2

Cessna Service Bulletin SEB01-2 Rev 2 (June 4, 2007)

-Covering actuator P/Ns 1281000-1/-2, 1281006-1/-2, 9882000-1/-2, 9882015-1/-2, 1241619-2, 1281001-1/-2/-3 for modification

-Covering 1281000-3/-4, 9882000-4/-5, 9882015-4, and 1281001-4/-5/-6 for inspection

These notices did not cover the actuators previously examined by the NTSB, though AD 76-04-01 covered the part in this investigation, having P/N 1280501-1. The markings on the actuator were consistent with it having been refurbished.

A postaccident examination of the landing gear was performed by a certified airframe and powerplant mechanic. He stated that the landing gear doors were locked in the open position. Removing the panels revealed that the left main gear rotary actuator was cracked and was leaking hydraulic fluid (see Figure 2).

Figure 2: Cracked actuator and location in the airplane.

The Electrol Inc. actuator, part number (P/N) 1280501-1, was sent to the National Transportation Safety Board (NTSB) Materials Laboratory for examination. The examination revealed that there was a crack through the barrel, with circumferential and longitudinal sections. The crack initiated at the retaining ring groove, where corrosion pits were present, and propagated via fatigue circumferentially (see Figure 3). The longitudinal portion of the crack, which extended through more than half the actuator barrel's thickness, displayed signatures consistent with tensile overstress fracture. The actuator was composed of aluminum alloy, and its hardness and conductivity were consistent with the expected material properties.

Figure 3: Crack location on the actuator barrel.

Scanning electron microscopy (SEM) examination confirmed that the fatigue cracking had originated at corrosion pits along the ring groove. As demonstrated by striation features, the fatigue crack propagated circumferentially around the barrel until it reached a critical size, and then partially fractured longitudinally from overstress. Corrosion pits were present at the fatigue crack initiation points along the groove.

The material examination of the actuator using energy dispersive x-ray spectroscopy (EDS) found the retaining clip groove contained remnants of the cadmium plating from the clip, and confirmed the presence of aluminum oxide at the crack initiation sites. These findings were consistent with fatigue failure due to the presence of corrosion pits and the progressive propagation of cracks under cyclic stress, leading to the actuator's failure and the release of hydraulic fluid.