Our aircraft electrical systems are becoming increasingly sophisticated and with it more complex. Understanding how it all works is beyond most of us so Steve Ayres suggests knowing what to do when something fails has never been more important…
Steve Ayres
29 September 2021
It’s hard to deny the capabilities of modern instrument panels. All-singing, all-dancing, reliable and lightweight, but when something goes wrong how does it tell you and do you know what to do about it? Even simple aircraft have increasingly complex electrical systems and rely on electrical power for everything – in some cases even to keep the engine running! So when there is a failure of some kind, then spotting that failure is vital, diagnosing it essential and understanding the consequences absolutely key to ensuring your flight ends safely.
Two recent incidents show how far reaching an electrical failure can go… and how serious consequences of misdiagnosis can be.
The pilot and passenger intended to fly to Southampton Airport where G-HAKA, a DA42 NG, was required for aerial work. The DA42 NG is a four-seat, twin-engine aircraft equipped with two Austro diesel engines and is equipped with a Garmin G1000 glass cockpit display suite. The aircraft is equipped with mission equipment specific to the aircraft’s role, and which increases the loads on the electrical system.
“The electrical load was further reduced by switching off the lights apart from the strobes”
Neither engine would start, and the pilot reported no glow-plug indications and poor engine turnover when using the starter motors.
Engineers from the operator’s maintenance provider eventually started the engines with the aid of a ground power unit. Engine ground runs and a download of the engine Electronic Control Units (ECUs) showed no anomalies.
A maintenance release form was signed off on the basis that the main aircraft battery charge state was probably low, and the pilot and passenger reboarded the aircraft behind schedule. The pilot reported that the engines started, but the low volts cautionary alert was displayed shortly after the cockpit checks were complete. This was disregarded ‘as a likely result of the earlier problems and something that would clear with engine running’.
Following a normal take-off there was an audible alert and the pilot again observed low voltage cautions for both main electrical busbars.
The pilot decided to return to Leeds Airport, but did not declare an emergency because ‘the checklist suggests 30 minutes time available’.
The abnormal checklist for low volts refers to two more checklists.
The pilot reported that the checks required by 4B.3.7 were completed, but not those in 4B.4.6 because an alternator failure was not displayed. The first action in checklist 4B.3.7 is to ensure that the alternators are switched on.
Following the checklist, the pilot reduced electrical loads but the busbar voltages decreased to 21 volts as the aircraft was on the downwind leg. After completing the pre-landing checks the pilot used a GPS tracking device to send a message to the operator informing them of the problem and selected the ILS approach plate on an electronic tablet. The electrical load was further reduced by switching off the lights apart from the strobes. As the aircraft started the turn onto the base leg the busbar voltages decreased rapidly and the pilot informed ATC that communications might be lost.
Shortly after settling on the ILS, all electrical power was lost, which resulted in the loss of the electronic flight displays. The pilot switched on the emergency power, one-shot battery for the standby artificial horizon, levelled off, and started a turn to the right. The cloud base was approx 700 to 800ft agl so, when a suitable gap was found, the pilot descended while maintaining sight of the ground. With the aid of a mobile phone-based flight planning application, the pilot was able to return to the airport where an uneventful landing was carried out.
The most likely scenario is that the main aircraft battery charge state was initially low, which meant the pilot was unable to start the engines.
The maintenance provider subsequently started the engines and switched the alternators off when they downloaded the ECUs, but this was not a requirement of the download procedure in the AMM. It was noted that five other procedures in the AMM contained cautionary notes that the alternators should be switched off during an event log readout, which is what the engineers said the aircraft manufacturer had taught them to do. The AAIB believe that the contradicting cautionary notes can cause confusion and, potentially, aircraft system damage.
The pilot reported that the low volts cautionary alert was displayed shortly after the engines were started and the cockpit checks were complete. This was dismissed as being associated with the earlier problems. Shortly after take-off, the pilot heard an audible alert and observed that both low volts indications were displayed. It is not known if the low volts indications cleared prior to take-off, but checklist 4B.3.7 stated that if the fault could not be rectified on the ground that the flight should be terminated.
The pre-flight and engine start checklists include checks that the alternator switches are on. The pilot reportedly checked the switches on two occasions, believing them to be on when they were off. It was stated that the alternator switches are normally left in the on position because the AFM does not require them to be switched off after flight. It is, therefore, probable that confirmation bias resulted in the pilot seeing what they expected when the switches were checked.
The G1000 display system does not depict the alternator switch status and has two voltmeters, one for each of the main busbars. If the alternator switches are off, the voltmeters continue to show the busbar voltage, which will be the main aircraft battery voltage. Furthermore, if the alternator switches are in the off position and the engines are running, the ammeters will show a current demand because the alternators will be supplying electrical power to the alternator control units, fuel pumps, ECUs and ECU back-up batteries.
This could potentially reinforce the belief that the alternators are switched on when they are off. The electronic display suite shut down when the main aircraft battery voltage reduced below the requirement to power the system.
The engines continued to run because the alternators continued to supply electrical power to the alternator control units, fuel pumps, ECUs and ECU back-up batteries.
The Tecnam P2008 was in cruise flight when, according to the pilot, the electronic flight display system ‘went out’. About 10 minutes later, the engine sputtered and lost power.
The pilot switched fuel tanks and attempted to restart the engine unsuccessfully then made a forced landing to a field. During the landing rollout, the aeroplane’s nose landing gear struck a ditch and it nosed over, resulting in substantial damage to the wings, vertical stabiliser, and rudder. Review of recorded data from the aeroplane’s instrument panel-mounted display system indicated that electrical bus voltage started at 12.6 volts when the system was powered on, decreased for the entirety of the recording, decreasing more rapidly near the end of the flight.
The alternator electrical current was zero for the entire recording.
Post-accident examination and testing of the aeroplane’s electrical system revealed that the system was functioning normally – all cautions and warnings, including the ‘ALT OUT’ message, were displayed by the system as required. Additionally, testing of the aeroplane’s engine and fuel pumps showed they also operated normally.
Given this information, it is likely that for the entirety of the accident flight, electrical power was being supplied by the aeroplane’s battery alone and was not being replenished by the alternator.
While the aeroplane was equipped with two fuel pumps, both required electrical power for operation. One fuel pump was likely not operating at all for the flight, as it required electrical power solely from the alternator for operation. The remaining fuel pump, which was powered by the battery, likely ceased operation as the battery voltage decreased to a level below that required to sustain it.
With both fuel pumps offline, the engine was subsequently starved of fuel, and lost total power. An abbreviated, six-page checklist was found in the accident aeroplane.
Comparison of the checklist to the published flight manual revealed differences from the Normal Procedures section in the flight manual, including no mention of checking the position of the alternator side of the split master switch on the checklist. It is likely that as the pilot began the flight, he did not activate the aeroplane’s alternator, nor did he notice the warning messages and voltage highlights that were called out by the aeroplane’s electronic flight display system during the flight that would have warned him of this omission.
Back in the days of yore, electricity was one of those things us GA types only knew was essential when the sun set. But now it is everywhere in our aircraft, and understanding the systems, and often more importantly, how they interact, has become a bit of a mind bender. In some ways, the growth in complex ‘plug and play’ replacement instrument panels and those now found in many homebuilts, has made understanding what’s going on more tricky.
Running out of electrical power, for whatever reason, can have implications for engine ignition systems, fuel pumps and most of our instrumentation (including all the engine instruments), not to mention the old classics of warning lights and horns, undercarriage and flap actuation.
In the first incident, believing that the alternator switches are never turned off, probably resulted in the pilot failing to recognise they were indeed off.
And despite all the audio warnings he remained convinced the cause was a low battery charge at the start of flight. Combine that with ammeters which showed current was flowing (in this case to the ‘essentials’ such as the engine control units) despite being turned off, and what the systems are actually ‘telling you’ is no longer straight forward, sometimes contradictory and ultimately confusing. This is not an unusual scenario and processing all the information to arrive at the right set of actions takes practise, familiarity and, yes, probably some form of simulation.
Having spent a fair bit of time in complex single-seat cockpits, sorting the wheat from the chaff was only ever possible through familiarity with the failure modes, and what needed to be done to ensure safe flight could be continued.
In both these incidents, a failure to switch on the alternators was nearly disastrous. For the Diamond, despite having one of the most modern and capable flight decks, the pilot was left relying on a one-shot battery for an attitude reference and a mobile phone for navigation.
In the second, power failure meant no fuel pumps and ultimately engine failure. Granted both incidents were as a result of our old friend human error in failing to switch on the alternators. But not identifying the error from the events that followed is a big worry and a reminder to all of us to brush up on the indications of alternator failure, to learn the immediate drills, and to understand the consequences of power loss.
