DA62 Forced Landing After Double Engine Shutdown Due to Multiple Electrical Issues (N84LT, Dallas, TX)
On 15 October 2022, private Diamond Aircraft DA62 N84LT made a forced landing near on West Kiest Boulevard near Dallas Executive Airport (RBD), Texas. The aircraft suffered substantial damage but the pilot and passenger were uninjured.
Diamond DA-62 N84LT Wing Damage After Forced Landing on Road (Credit: FAA via NTSB)
The US National Transportation Safety Board (NTSB) released their safety investigation report on 26 November 2024.
The Accident Flight
The NTSB explain that the aircraft…
…departed Winston Field Airport (SNK), Snyder, Texas, about 1448, and climbed to a cruise altitude of 11,500 ft mean sea level (msl).
While descending in preparation to fly a visual approach [at RBD], the pilot lowered the landing gear and contacted the tower controller for landing clearance.
During the controller’s response, the pilot reported hearing a “pop” and observed the avionics display screens lose power. A few seconds later, he observed that both engines had lost power.
The pilot executed a forced landing on to a road, during which the airplane struck a power line in the descent, and after touchdown, two road signs, which resulted in substantial damage to the right wing.
Safety Investigation
The DA-62 was powered by two Austro Engine E4P-C liquid-cooled, in-line four-stroke four cylinder diesel engines. These each has two Electronic Control Unit (ECU) powered by an alternator when at least one engine is running. When both engines are off, the ECUs receive its their electrical power from either the main battery (a 24 V, 13.6 Ah lead-acid battery) or from back-up sealed-lead-acid ECU batteries. The back-up batteries give 30 minutes of electrical power.
Investigators say a loss of main battery power…
…most likely occurred during the landing gear extension when the hydraulic pump turned on, and simultaneous with the radio transmissions.
Investigators also found that the electrical connector to the hydraulic pump pressure switch was damaged.
Photo of the damage to the pump pressure switch electrical connector. (Credit: Austro Engines via NTSB)
NTSB do not comment on the possible source of damage or when it may have occured.
Furthermore:
At both alternator relays, the wiring was incorrect. As wired, the alternator relays would NOT disconnect the alternator power from the main electrical system. The relays would cut power to the glow plugs for the respective engines (…glow plugs are only used during engine start). In addition, the 80 Amp fuse was not between the alternator and the aircraft electrical distribution system.
Drawing D67-9224-30-01_01 mark-up showing actual alternator relay wiring (Credit: Diamond Annotated by NTSB)
However, during the aircraft and component testing a definitive root cause of the initial power failure could not be determined. Testing could not duplicate the conditions of the flight when the electrical system anomaly occurred or the anomaly itself.
Two battery system issued were identified:
- The main battery had been installed 23 days earlier. The aircraft had flown c 15 hours since. When its capacitance was tested by investigators it was found to be at 81.2% of rated capacity, below the >85% requirement.
- Four new ECU backup batteries had been installed during an annual inspection at a facility in Texas about 4 months earlier. Investigators found that these were incorrectly wired in parallel rather than in series. The in-line fuses for the backup ECU power system for both engines were found to be blown.
In relation to the ECU backup batteries:
The incorrect wiring would have resulted in only 12 volts instead of 24 volts being available which would have resulted in a higher current draw and subsequent blown fuses.
The battery issues identified resulted in the engine shutdown after the prior electrical system failures.
Looking more closely at the wiring errors, investigators found that:
A review of the aircraft maintenance manual showed that it did not provide a procedure to verify the ECU backup batteries were functioning correctly after replacement.
The last step in the ECU back-up battery installation was to run engines and verify that the electrical system operated correctly.
This step does not actually verify that the ECU backup battery installation because unless there is a wider electrical system failures, the ECU backup batteries are not called upon to function.
In addition, there was no ECU backup battery operational test. If there had been such a test, the improperly connected batteries would have been discovered immediately after replacement.
Completing this check on a routine basis would [also] determine if the in-line fuse has blown.
Furthermore, the pilot did not have the ability to check for the proper functioning of the ECU batteries before every flight during the before-takeoff checklist.
Diamond does however complete a test of the ECU back-up batteries as part of their new-build production process.
NTSB Probable Cause, Safety Action & Our Observations
A total loss of airplane electrical power for undetermined reasons, which resulted in a complete loss of engine power to both engines. Contributing was the worn main battery and the incorrectly wired ECU backup batteries.
We would observe that limits to the maintenance data, particularly in relation to functional testing, contributed to the failure to detect the incorrectly wired ECU backup batteries, the final line of defence before an uncommanded engine shutdown.
After this accident, Diamond Aircraft published Service Information Letter No. SI62-034 – Inspection of ECU Backup Batteries Wiring Installation on all DA-62 aircraft. This verified the ECU back-up batteries were connected correctly and the in-line fuse was intact.
This was issued by Diamond very swiftly on 17 November 2022.
The absence again in an NTSB report of any attempt at maintenance error investigation, even attempting to interview the personnel involved, is again concerning and limits the ability of the NTSB to achieve their goal of preventing future occurrences.
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