Electrical power alternatives essential in the digital-instrument age

KEEPING THE JUICE FLOWING

STORY BY– Dave Higdon

Source: Avionics News, February 2018

There's a truism that pilots recite during hangar-flying sessions and other discussions that holds some truth for their electrical systems. The line goes, “The only time you have too much fuel is when your plane is on fire”. For the electrical system, we upend the truism: There's almost never a time when you have too much juice.

Sure, there are overvoltage situations that can burn out electrical components – but our aircraft typically use protections against those situations in the form of circuit breakers, fuses and electrical crowbars designed to cut all power from the electrical system.

That’s not the same as having multiple options for keeping electrical power flowing to critical instruments, communicators and navigators. And in today’s world of all-glass cockpits and digital instrumentation, multiple options become a necessity rather than a luxury.

It boils down to this: No juice to power those screens, no information on the displays; no radios; no navigators; and no powerplant status information. Today, some glass- panel converts opt to retain some analog instruments. For IFR digital cockpits, the Federal Aviation Administration likes to see indications for, at a minimum, the pilot's information Triple A: airspeed, altitude and attitude.

Options exist for independently powered digital versions of the Triple A; options for three-packs of analog instruments, with the airspeed and altitude plumbed into the pitot-static system and a built-in battery to drive the attitude gyro.

And some aviators opt to stick with the same information format as their primary flight display by installing a smaller version of the standard PFD - either with its own battery power or powered by a secondary, or standby, electrical power source.

But alternative power solely for the flight instruments doesn't automatically cover emergency power for the rest of the panel – the radios pilots depend on for communication and navigation.

Standby electrical-power solutions can cover that need and came on the scene decades before digital cockpit instruments. In fact, the majority of Part 23 aircraft can claim their systems provide an inherent alternative to the electrical-generating source. And most pilots flying analog panels continue to cruise along relying on the main-ship battery as their standby electrical-power source. But that status depends on the split power configuration of most light aircraft's flight instruments: two air-driven analog gyros - the attitude indicator and directional gyro - and three air-driven air-data instruments, airspeed indicator, altimeter and vertical speed indicator. None of them need electrical power to function; and a single electrically driven turn gyro.

Lose air power and the turn gyro continues to spin on electrical power. Lose the electrical-generating source and the airpowered five continue to work - as do all the electrically powered components, but only as long as the battery lasts. Everything running off the fades from utility as the components drain the battery.

The same can't be said for today's growing population of alldigital panels, stacks flying in aircraft with no suction or airpressure source because they use no air-driven instruments. Even the readouts from the pitot-static system are electronic translations of air pressure on solid- state - that is, electronic - air-data sensors.

Generically, we call it an electronic flight information system, or EFIS. For operators flying aircraft transitioning to glass panels, alternative electrical power is a must - if not implicitly by regulatory fiat, by dent of survival instincts and the reality that a main- ship battery may be insufficient to keep a glass panel and radios functional long enough to get the plane on the ground after a total electricalsystem failure.

With retrofit spending leading forward-fit in avionics-industry sales according to the latest AEA Avionics Market Report, it appears more pilots continue to turn to digital-options panels for their planes.

Thankfully, the avionics industry provides multiple options to keep power flowing to critical displays, communicators and navigators.

IT BOILS DOWN TO THIS:

NO JUICE TO POWER THOSE SCREENS, NO INFORMATION ON THE DISPLAYS; NO RADIOS; NO NAVIGATORS; AND NO POWERPLANT STATUS INFORMATION

The typical analog-panel power system

In the majority of single-engine aircraft, some form of lead-acid or, more recently, lithium-ion battery, serves two major purposes: power to start the engine; and as the standby electrical-power source in the event of the loss of electricalgeneration capability – a failed engine-driven alternator or generator.

Power to recharge the battery and to run radios and instruments is routed to a main-electrical-system bus and through protective breakers or fuses to the various devices powered by the electrical system.

Lose generating power and the load shifts automatically to the main-ship battery. In the era with split analog flight-instrument power, the main-ship battery provides power sufficient to run a radio, a navigator and transponder, and, typically, the turn coordinator.

As noted before, the flow of air or air pressure spins the two remaining gyro flight instruments, the AI and the DG, also known as a gyro compass. Electrical power became a morecritical issue in light aircraft when EFIS cockpits with electronic displays began to displace analog panels.

Go to glass from spinning-mass gyro instruments and the FAA wants aircraft to employ an alternative: either an alternative, stand-alone instrument with its own independent power source or a second source of power to keep the EFIS components functioning after losing main-ship power.

In place of spinning-mass gyroscopes and hollow-tube analog air-data gauges, these panels employ devices known as MEMS, or micro-electro-mechanical systems - basically, a miniaturized mechanical and electro-mechanical sensor capable of sensing motion and pressure.

MEMS provide the air- and attitude-data sensing found in modern primary flight displays via the AHRS, or attitude and heading reference system. By orienting MEMS sensors differently, the AHRS senses movement in three axes, providing attitude information on roll, pitch and yaw.

ADAHARS, or air data and attitude heading reference system, add air-data measurements to the mix to display airspeed, altitude, and rate of climb or descent in addition to the attitude indications. The sensors and computer chips convert sensor output into information displayed in screens. And all components in this process, from MEMS to PFD and multifunction displays all require electricity - hence the increased emphasis on electrical-power continuity.

The FAA published two advisory circulars that cover the subject: AC 23.1311-1C Installation of Electronic Display in Part 23 Airplanes; and AC 23-17C, Systems and Equipment Guide for Certification of Part 23 Airplanes and Airships.

The standards per the FAA

Just as options exist to retrofit existing aircraft with glass and digital components in place of their analog avionics stacks, options exist to enhance electrical systems and improve standby capabilities. And what we discuss here centers on what's necessary for Part 91 operations of Part 23 aircraft. Part 23 aircraft used in Part 135 charter ops face a higher bar.

Part 23, § 23.1353(h) requires that: “In the event of a complete loss of the primary electrical power-generating system, the battery must be capable of providing at least 30 minutes of electrical power to those loads that are essential to a continued safe flight and landing. The 30-minute time period includes the time needed for the pilots to recognize the loss of generated power and take appropriate load-shedding action.”

Part 135 operations flown in Part 23 aircraft requires that single-engine aircraft have:

1. Two independent electrical power-generating sources, with each able to supply all probable combinations of continuous inflight electrical loads for required instruments and equipment; or

2. I n addition to the primary electrical power-generating source, a standby battery or an alternate source of electric power that is capable of supplying 150 percent of the electrical loads of all required instruments and equipment necessary for safe emergency operation of the aircraft for at least one hour.

For the pilot operating under Part 91, though, the minimum is seldom enough. And that battery capable of “providing at least 30 minutes of electrical power to those loads that are essential to a continued safe flight and landing” in an analogpaneled aircraft probably won't make that 30 minutes driving the higher electrical demand of an all-glass, all-electrical panel.

Consider these approaches to assuring power for allelectrical aircraft.

THE GOLD STANDARD: STAND-ALONE STANDBY ELECTRICAL POWER

Original equipment manufacturers offered dual alternators and, in some cases, dual batteries going back decades, years before anyone in general aviation uttered the words “primary flight display.”

With digital cockpits the dominant standard in today's factorynew aircraft, electrical power redundancy has become more common. The catalyst? The need for redundancy to power allglass instrument panels.

You'll find dual-electrical systems like this as standard on many factory airplanes with EFIS cockpits: Beechcraft G36 Bonanzas and G58 Baron twins; Cirrus Aircraft SR20 and SR22 models; current production Mooneys; even the modest Cessna singles, the 172,182 and 206 sport dual buses.

Many of these sport dual everything: alternators, to produce electrical power; batteries and regulators, to control the output and sharing of power from the two alternators; and dual buses.

For glass-panel aircraft lacking these capabilities, avenues exist for some aircraft to add them.

Depending on the airframe, supplemental type certificates may provide the path to adding a second alternator and a regulator capable of handling both the main and second generating source.

Several vendors offer packages that add a standby alternator, some driven off the engine accessory drive normally used to power the now-unneeded suction or pressure pump, along with the regulator capable of controlling both - including picking up the load when the main alternator fails.

B&C Specialty Products offers standby alternator packages for many popular aircraft, including a regulator capable of handling the changeover. Both belt-driven and gear-driven options are available, and the company holds STCs for many popular aircraft. B&C also holds STCs and offers kits to fit its standby alternator systems in various aircraft, among them the Beechcraft Bonanza, Cessna 210 and Piper PA32.

Basic Aircraft Products also offers STCs for its ram-air turbinepowered standby alternators for several airframes. In essence, the ram-air turbine pops out where airflow through turbine blades drives an alternator.

Fourteen-volt systems available from BAP include most Aeronca models, short-wing Pipers through the Super Cub, Taylorcraft BC 12-D, the Luscombe 8 series and Boeing Stearman.

Another avenue for adding alternate electrical power involves using the field approval approach to add a second bus and a standby battery. But other avenues exist to assure power continues to flow to digital panels.

EFIS with integral standby power

In many instances, avionics manufacturers offer integral standby power in or for today's modem EFIS gear.

Avidyne, Aspen Avionics, Dynon and Garmin offer the option of a standby battery for their systems.

For example, Aspen's Evolution line of PFDs and MFDs come standard with both a standby battery and a GPS navigation receiver. For VFR-only pilots, Aspen's certified VFR PFD offers an inexpensive path to adding modern solid-state indications for the entire six-pack of analog flight instruments in a standard panel for about $4,000 plus installation. And that includes the standby battery and GPS.

Aspen's higher IFR-approved PFD and its line of MFDs also include the standby features as standard - and the battery satisfies the regulatory requirement for that instrument.

Garmin offers the option of a standby battery for both its new G600 TXi and G500 TXi PFDs, also satisfying the emergencypower requirement. Ditto for Garmin's G5 instrument, which is approved as a replacement attitude indicator and, in a second unit, a DG and HSI - with a GPS interface as an option. The G5 can be equipped with its own integral standby battery to keep the units working for more than two hours in the event of the loss of main- ship power.

And Dynon's new systems for certificated aircraft also have the option of standby power.

Mid-Continent Instruments' True Blue Power division offers standby battery products capable of powering many panels, as well.

Operators of experimental aircraft enjoy many more options for standby power from most of the vendors supplying IFReligible glass-panel components - Dynon and Garmin among them. Avionics with integral standby power provide a largely seamless changeover to the battery, requiring no action on the part of the pilot, along with an indication of the changeover.

As attractive as these options may be, however, they don't provide power to other avionics in the stack, such as VHF communications and navigation radios or GPS navigators and transponders.

PRESERVING POWER FOR THE FULL PANEL

A second alternator and/or battery offers the best option for keeping the entire panel powered in the event of a total electrical-system failure. Maintaining glass PFDs is critical; but in an emergency, retaining the ability to navigate and communicate with air traffic control hold equal importance.

And it's here where some adjustment to power buses may be necessary. For example, some aircraft with standby power as standard employ a second standby bus that powers a reduced amount of equipment to keep the load within the limits of the standby alternator.

That option should be considered as a must when adding a standby alternator with lower output than the main alternator.

And it's here where it's advantageous to consider opting for all the standby battery options of the digital panel components as well as a standby alternator. With all the PFDs and MFDs capable of running on their own internal batteries, the power from the standby alternator can be dedicated to powering the other avionics - particularly the transponder, ADS-B gear, a GPS and a comm radio.

Of course, matching main-ship power to the higher electrical demands of an all-glass cockpit is fundamental to upgrading to EFIS panels. Likewise, managing the load carried by standby systems is equally necessary to avoid exceeding the capabilities of the altemative-electrical-power system and possibly bringing on a second power loss.

For the pilot facing an in-flight power loss, this goes a long way to assuring the flyer must face only one emergency at a time.