Peter wrote:
The Q I see is whether the alternator torque is relevant in the wider picture of alternator failures, including alternator drive belt (or plastic coupling, if used instead of a belt) failures.
Personally (as written already), I have never experienced this kind of failure (although I have flown several thousand hours on types with gear driven alternators). The most common failure has been the electronics of the voltage regulator or paralleling unit (in twins). Brush failures are almost unheard of with aircraft that are maintained under a maintenance program (e.g. CAME) because they get replaced in regular intervals regardless of wear.
Turning off the generator at low RPM during approach will certainly reduce the torque of the generator drive. But as written by Archie above, switching it on again at even lower ground idle RPM after landing will put a significantly higher torque on it. The partially depleted battery (drained during that generator-less approach) will demand a lot of charging current – certainly more than what can be saved by switching the landing light off.
And regarding Pilot DARs “certification requirement” of 30 minutes battery life: The only such requirement I am aware of is that on part 25 (transport category) aircraft the standby instrument power must last 30 minutes or more. The main battery (as demonstrated to me on several occasions in my simulator training) lasts between 5 and 10 minutes only. After my (only one to this day) dual generator failure in a light twin the battery lasted about five minutes. After that everything was dead, including the electric gear.
The 30 minute battery requirement:
Sec. 23.1353
Storage battery design and installation.
(a) Each storage battery must be designed and installed as prescribed in this section…….
(h) (1) In the event of a complete loss of the primary electrical power generating system, the battery must be capable of providing electrical power to those loads that are essential to continued safe flight and landing for:
(i) At least 30 minutes for airplanes that are certificated with a maximum altitude of 25,000 feet or less; and
(ii) At least 60 minutes for airplanes that are certificated with a maximum altitude over 25,000 feet.
(2) The time period includes the time to recognize the loss of generated power and to take appropriate load shedding action. Quote
When doing the full electrical load analysis of a standard Cessna Caravan, the math showed that the battery endurance was 30.49 minutes, so not a lot left to play with for our mod. Thus, I tend toward recommending automatic load shedding systems for our non essential equipment – no pilot intervention required, so no time penalty. Depending upon the indication to the pilot of the electrical failure, you can add as much as 10 minutes to that 30 minutes, to allow for the pilot to recognize the generating failure. That can be very penalizing to an already endurance challenged system!
My preference to use battery only at idle with a high current draw was based upon 20 amps of landing light, plus 6 amps of nav lights, plus 7 amps of flashing beacon, plus whatever else was turned on for a night landing. The draw was in the 35 amp range during approach. After landing and taxi in (on the battery only), lights could be turned off, and the engine power increased a little. The draw to top off the battery was usually 10 to 15 amps for a few minutes, so much less than the draw at idle power. It’s easy to watch the charging progress on the digital ammeter. Since installing LED external lights, this is much less a concern, and I fly around lighting up the world, without a care.
The risk of sudden failure while operating on the battery alone is very low, so “redundancy” is of no concern to my in that regard. It’s just slow discharge which is a concern to me. I would be very much more worried about completing a longer night flight after an alternator failure, and no charging ability, than I would be about the last stages of an approach and landing on battery only, when turning a serviceable alternator back on is a switch flick away.
Of course, if the flight manual for the aircraft states no to turn off the alternator, don’t do it! (but Cessna 100 series have no such warning) – It just shows our increasing dependence upon electricity in our equipment! Yes, I used to own a camera which did not use batteries at all. You had to think more to use it though!
Pilot_DAR wrote:
the battery must be capable of providing electrical power to those loads that are essential to continued safe flight and landing
This I think is the key phrase. What electric system is essential for safe flight? COM 1 maybe and an electric turn coordinator as backup for pneumatic attitude instruments. But on a glass cockpit plane with all electric instruments? I strongly doubt that this 30 minute requirement is really met by many modern types.
what_next wrote:
What electric system is essential for safe flight?
‘Depends what the plane is certified for. If night IFR, it’ll be all the lighting, pitot heat, and avionics required for that. If day VFR, it’ll be a much shorter list of equipment. The flight manual will state what you can shed, and what you must keep on, and that list will correspond to the demonstration of compliance to the requirement.
For myself, the last time I had an alternator failure (really, the voltage regulating part of the system) I flew most of the way home (uncontrolled airspace) with the master entirely off, so I knew that I would have power for the last portion of my flight, which would be a dusk landing, as long as I elected to continue, which I did.
You can be sure that the modern certified glass cockpit aircraft meet this requirement – of they would not be certified. But, they might not surpass the requirement by much!
