Practical tips on alternator failure (May 2005)
Rather than continuing to gripe about the inadequacies of
the electrical system in our airplanes, I thought it might be helpful for
pilots to work through the myriad issues that arise when Alt 1 fails from the
convenience of their home. This way perhaps it won’t be a stiff challenge when
this situation arises in the air. Bear in mind that an Alt 1 failure can be
extremely challenging in either VFR or IFR conditions, depending on the
remaining time in flight, so advance preparation and familiarity with the
situation is crucial.
Like the precautionary words on icing (“land now”) the POH does not offer much in the way of “news you can use” when it comes to anticipating and dealing with an electrical system failure. Sure, “land now” is prudent advice, but that isn’t always practical.
Here are my perspectives on this issue:
1. Pre-flight preparation: Set up your avionics so that it is easier to identify and cope with a failure of Alt 1. Specifically:
a. Put the “Switch Fuel Tanks” message onto Garmin 1, since you can’t expect to have Garmin 2 or the analog fuel gauges available after an Alt 1 failure. Or keep track of fuel tank changes using paper and your watch.
b. Ensure that the flight plan is loaded onto Garmin 1, since you can’t expect to have Garmin 2 available after an Alt 1 failure. I’m sure virtually all pilots already do this, but one can’t be too sure.
c. Set up the data blocks on the Map page of the MFD to include Volts: this will display the voltage for the main bus and the essential bus. When Alt 1 falters, you’ll see the first number (main bus) drop and turn yellow as Bat 1 takes over and provides only 24 volts instead of the 28 volts that are normally supplied by Alt 1. Sometimes this happens minutes before the annuciator light comes on, but it’s a sure sign of an impending Alt 1 failure. The Emax page provides the same information.
d. Set the ammeter gauge to the middle setting (BATT). In normal operation, the AMP side of the gauge (the right dial) will show zero. When Alt 1 fails, the gauge will show a negative value as Bat 1 begins to drain and it becomes the sole source of power to the main bus.
2. Signs of failure: The POH is complete in discussing how to detect an Alt 1 failure. The annunciator will light up “Alt 1”; the volts reading on the Emax or Map pages of the MFD will show less than 28V for the main bus; and the ammeter will show a negative value for Amps when the toggle switch is in the middle setting (BATT).
3. Troubleshooting: The POH and emergency checklist are complete in this area as well: turn off the Alt 1 master switch, pull the Alt 1 circuit breaker, reset the breaker, look for the annunciator light to go off and for the volts reading to show 28V on the main bus. Good luck: I’ve never heard of a false Alt 1 failure. Once the annunciator comes on or the main bus shows less than 28 volts, the alternator is already non-functional and no amount of breaker-pulling is going to bring it back to life. Therefore I wouldn’t spend alot of time on this troubleshooting exercise.
4. Take action: You’re now reconciled to the reality that Alt 1 is gone. Its time to Aviate, Navigate and Communicate. So make sure:
a. The autopilot is on
b. The flight plan on Garmin 1 is current and that you’re navigating directly to the correct waypoint
c. Set Garmin 1 to the map page (NAV page two) either in track up or north up mode (strictly a matter of personal preference)
d. Scroll the Garmin map to an appropriate distance so that you can confirm the situational awareness that will be lost when the MFD goes blank.
e. Follow the Decision Tree below and choose a destination airport.
f. Call ATC and advise that you “need to divert to the (new) destination airport due to an electrical system failure”. If the Decision Tree favors Conserve mode (i.e., turning off Bat 1 and pulling many circuit breakers), tell ATC that you “will soon lose your transponder, due to the need to conserve battery power for approach and landing.” FYI, in the aftermath of September 11, even without a transponder ATC will still be able to see your plane as a primary target, but expect to be asked to report reaching a given altitude or fix.
g. If the Decision Tree favors Conserve mode (by turning off Bat 1), be aware that the audio panel will no longer function. This means that ANR headsets (e.g., Bose) will only work out of one ear in mono mode, and it will be impossible to talk to passengers. The MFD will be gone, as well as Garmin 2, Traffic Watch, Stormscope, cabin lights and the accessory power outlet. Don’t expect to use these devices for the remainder of the trip.
5. The Decision Tree: The POH is not helpful in analyzing which circuit breakers to pull in order to shed load once Alt 1 is gone. Nor does it adequately describe what will happen if you don’t switch to Conserve mode by turning off Bat 1 so as to save its precious power for when it will be needed. These are critical decisions that need to be analyzed in order to be able to safely fly the plane and land without undue complications or pilot workload. This section outlines the decision tree that I’ve developed to maximize available electrical power and minimize pilot workload and complexity for the remainder of the flight. The key question to ask is: What is the remaining time to the original destination?
a. If the remaining time to the original destination is less than 30 minutes and VMC prevails for the remainder of the trip, then Conserve mode is not necessary. So leave Bat 1 on and fly normally. You will have all of the capabilities of the plane for the remainder of the flight.
b. If the remaining time to the original destination is less than 30 minutes and IMC prevails for all or part of the remainder, consider using Conserve mode by shutting down Bat 1 until the approach/landing phase of the flight. This will conserve the electrical power necessary to safely fly the plane in case you end up with a missed approach and need to spend additional time flying to an alternate airport. See Section 6 below for a discussion of which circuit breakers to pull so that Conserve mode is in effect when Bat 1 is turned back on.
c. If the remaining time to the original destination is more than 30 minutes, you should consider diverting to a closer airport. If one is available and VMC prevails, then Conserve mode is not necessary. So leave Bat 1 on and fly normally. You will have all of the capabilities of the plane for the remainder of the flight.
d. If the remaining time to the original destination is more than 30 minutes but there is no suitable alternate airport, you MUST enter Conserve mode. Shut down Bat 1 regardless of whether its VMC or IMC if you want a safe and uncomplicated flight. See Section 6 below for a discussion of which circuit breakers to pull so that Conserve mode is in effect when Bat 1 is turned back on, as may be necessary to change power settings, switch fuel tanks, deal with icing, or execute an approach or landing.
6. Conserve Mode: The main bus (controlled by the last two columns of circuit breakers) feeds many very important flight instruments and controls, so its a huge misconception to think that the Cirrus can be safely flown just off the power to the “essential bus” that is provided by Alt 2 and Bat 2. In other words, if Alt 1 fails but you don’t turn off Bat 1 shortly afterward, you won’t have a variety of important flight instruments and controls available for landing. These include: engine instruments, fuel gauge, flaps, pitch and roll trim, transponder, pitot heat, instrument and nav lights (night flight), TKS system, fuel pump (when switching tanks), and the strobe lights. My philosophy is to pull ALL of the circuit breakers on the main bus, EXCEPT the following:
Column A (Main Bus):
9 Engine instruments
10 Instrument lights*
11 Pitch trim
12 Roll trim
Column B (Main Bus):
1 Fuel pump
7 Pitot heat
10 Strobe lights
11 Nav lights*
Column C (Essential Bus): Don’t touch any of these circuit breakers
*These circuit breakers should be pulled if its NOT night flight or IMC
Once the appropriate circuit breakers have been pulled, you can turn Bat 1 back on for selected functions without having to worry about wasting power on non-important devices (e.g., Traffic Watch or Stormscope) that also occupy the main bus.
When it comes time to switch fuel tanks, turn on pitot heat or TKS, or change a power setting, switch on Bat 1 for the minimal amount of time that is necessary to accomplish those tasks. When it comes time to shoot an approach or enter the landing pattern, switch on Bat 1 so that power settings, flaps and trim can become functional. At all other times, fly the airplane only from Alt 2 and Bat 2, using only the autopilot, Garmin 1 and PFD to aviate and navigate. Forget about having the MFD, Garmin 2, transponder, Traffic Watch, Stormscope, audio panel, cabin lights or the accessory power outlet available once Alt 1 fails and Bat 1 and the main bus circuit breakers are set up for Conserve mode.