N406Z

On June 22, 2024, about 0835 mountain daylight time, an experimental amateur-built replica PA-12, N406Z, was substantially damaged when it was involved in an accident near Douglas, Wyoming. The pilot sustained minor injuries. The airplane was operated as a Title 14 Code of Federal Regulations Part 91 flight test.

According to the pilot, he was beginning Phase 1 flight testing following the installation of a new engine. Before departure, the pilot completed an engine run-up, during which time he cycled the ECU switches (the airplane was equipped with two ECU switches). The engine ran smooth and continuously when ECU 1 was switched off and ran rough when ECU 2 was switched off. The pilot then switched both ECUs to the ON positions and enrichened the mixture before departure to cool the cylinders while breaking in the engine. He cycled the ECUs one more time and received the same result, but determined the airplane was safe to fly with both ECUs on.

Most of the flight was uneventful. The pilot remarked that he maintained about 38% fuel trim to maintain the cylinder temperatures within a desired range. He also manually switched fuel tanks from the left tank to the right tank after 1 hour of flight time. At that time, ECU 1 indicated blue and ECU 2 indicated green (both were connected and operating: the green indication meant that ECU 2 was operating and connected to the rest of the system; the blue indications meant that ECU 1 was in control of the fuel injectors). However, at about 0815 one of the multi-function displays started to flicker.

About 20 minutes later the pilot flew over his destination airstrip at an altitude about 700 ft above ground level. As he approached the airstrip, the pilot configured the airplane for landing by establishing an airspeed of 65 mph, indicated with 2 notches of flaps extended. After he decreased manifold pressure on the downwind leg for the northwest runway, the engine lost all power. When the pilot looked at the display after the power loss he noted that the ECU 1 indicator displayed red and the ECU 2 indicator displayed grey (indicating that both ECUs were off). The voltage meter showed a battery strength of about 14 volts with no low power annunciations.

The pilot turned to the base leg of his approach and cycled the ECU switches; however, both ECUs repeated the same indications (ECU 1 red, ECU 2 grey). After he turned to the final approach leg he cycled the fuel pump, which operated properly according to the flight display, but observed no change when he cycled the ECUs a third time. The pilot selected a landing point on the runway and put the airplane in a slip momentarily to descend. He then rolled wings level when he was ready to make runway contact, and subsequently flared. The airplane touched down “moderately” hard with the wings level in a three-point attitude; however, both main landing gear axles failed, and the airplane nosed over, which resulted in substantial damage to the rudder and vertical stabilizer.

Data retrieved from an onboard panel display showed that the airplane departed Converse County Airport, Douglas, Wyoming, at 1235:15 and flew uneventfully for about 2 hours. At 0833:14, as the airplane was starting on the downwind leg of its approach, the engine speed was 2,400 rpm with a fuel pressure of 24 psi. The engine speed and fuel pressure then started to decrease and reached 1,170 rpm with a fuel pressure of 6.2 psi as the airplane started its base turn at 0834:01. The engine power advanced slightly to about 2,000 rpm with a fuel pressure of 11.9 psi and engine power decreased to about 670 rpm with an increase in fuel pressure to about 21 psi; at the time the airplane was on final approach at 0834:27. Over the next 20 seconds (to the end of the flight), engine power decreased to about 400 rpm with short, momentary advances in power; however, fuel pressure continued to indicate about 21 psi. No data was available from the ECU.

According to the pilot, he had selected and installed axles that were insufficient for the airplane. He told his mechanic to proceed with the installation and that he would “keep an eye on them,” contrary to his mechanic’s suggestion.

The airplane was equipped with an Aero Sport Power IO-375-EXP air-cooled, horizontally opposed, 200 hp, reciprocating engine.

The airplane also had an electronic fuel injection and ignition system manufactured by EFII. According to the company’s website, the system is a fully redundant electronic engine management system that could be used in any experimental category airplane equipped with four- or six-cylinder Lycoming engines. The website also describes the integration of the engine control unit with the engine accessories and fuel delivery system.

“The system is controlled by an ECU (the computer) and incorporates a fully mapped fuel curve and ignition timing curve. The electronic fuel injection portion of the system utilizes electronic fuel injector valves that are installed into each cylinder using our Port Mount Injector (PMI) system. A throttle body replaces the typical carburetor or mechanical injector butterfly assembly. The electronic ignition portion of the system utilizes high energy inductive ignition coils instead of magnetos to deliver a very strong and efficient spark to each plug. All System32 kits are fully redundant and include two System32 ECUs and redundant engine sensors.”

According to the ECU manufacturer, the fuel flow must be calibrated for accuracy, and they provide instructions to complete this task. The pilot’s mechanic reported that the pilot declined their offer to calibrate the fuel flow system before the first flight.

Testing data from the engine conversion showed a fuel flow of 7.7 gph at 2,200 rpm, 10.3 gph at 2,400 rpm, and 18.2 gph at 2,700 rpm. The pilot reported that he departed with 40 gallons of fuel onboard and subsequently flew for 2 hours before he experienced a total loss of engine power. Data captured by the engine monitoring system showed that the pilot maintained an engine speed of 2,500 rpm for most of the flight. Performance computations showed that the engine would have maintained a fuel flow of about 14.3 gph at this engine speed without fuel trim applied and 19.6 gph with 38% fuel trim, which was the trim used as reported by the pilot. A photograph of the fuel flow during a flight the day before showed 14.4 gph fuel flow at an engine speed of 2,500 rpm.

The ECU also uses a color-coded system displayed on panel in the cockpit to show the status of the ECU and its redundancy. According to the manufacturer:

· Green indicates that the ECU is operating and connected to the rest of the system

· Blue indicates the ECU is in control of the fuel injectors and connected to the system

· Red indicates that the ignition function (P-lead switch) is in the off position, but the ECU is still connected

· Grey indicates the ECU is not connected to the rest of the system, which could be due to a power loss or disconnected communications cable

The ECU manufacturer also noted that only one ECU can be in control of fuel delivery and that the ECU select switch manages which ECU is in control. A blue label on the ECU monitor indicates the ECU that is actively in control of fuel delivery and a green label shows that the ECU is not actively in control.

According to the ECU manufacturer, two engine run-up switches toggle each ECU between RUN and STOP mode (one switch controls ECU 1 and the other controls ECU 2). The ECU will show red if the engine is in STOP mode. The ECU manufacturer reported that the pilot would never select STOP mode in flight as these switches are designed to complete the run-up ignition test, or “mag check.”

The ECU manufacturer also reported that the system comes pre-programmed, but that the fuel flow on the ECU is not considered accurate until it has been calibrated.

Postaccident examination of the engine did not reveal any preimpact mechanical anomalies or malfunctions that would have precluded normal operation. All the electrical components of the EFII engine management system were secure at their appropriate terminal connections and found to be in serviceable condition. A postaccident engine run showed that with both ECUs in the ON position and the engine speed at 1,530 rpm, the ECU 1 indicator displayed blue and the ECU 2 indicator was green. When ECU 1 was switched off the indicator displayed red while ECU 2 displayed green. The engine was smooth and continuous at 1,500 rpm and when ECU 1 was switched on the engine continued to run smooth. When ECU 2 was switched off the engine speed dropped to 1,270 rpm. During this time ECU 1 indicated blue and ECU 2 indicated red on the controller panel. When ECU 2 was switched on the engine speed increased to 1,520 rpm and ran smooth. The engine was subsequently operated between 1,000 rpm (idle speed) and 1,800 rpm with no rough running indications or interruptions in power. The ECU indications did not change when the fuel was depleted and the engine ceased running.

According to the ECU manufacturer, a grey annunciation with one ECU and a red annunciation with the other ECU suggests there was likely a “global fault affecting multiple systems, such as a primary ground or power connection somewhere in the plane exhibiting an intermittency (loose connection, broken wire, etc).”

The ECU manufacturer also noted,

“During flight operations, the ECU labels would never be RED. They would only be GREEN or BLUE. If ECU labels are something other than GREEN or BLUE, this indicates a wiring or voltage issue.

The two ECUs have their own separate power and ground wiring and operate as separate systems. If both ECUs are showing status indications that are other than GREEN or BLUE, this indicates that both systems are seeing power faults of some sort.”

The ECU manufacturer also offered an opinion about the change in ECU labels. He opined,

“…this suggests there was an upstream cause of the power fault either on th...