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Posted for Brent Regan <brent@regandesigns.com>:
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2) The battery was charging at 10 amps, just fine.
You sure about that? Ammeters are usually on the alternator B lead so they
measure the output of the alternator. Your instruments and avionics would
take most of that 10 amps. Your battery probably wasn't charging at all.
Ammeters are a frequent source of confusion because pilots frequently do not
fully understand the dynamics of the electrical system. There are three main
factors that must be considered; the output of the alternator, the current
flowing into or out of the battery and the aircraft's electrical load
(excluding the battery charging). To know the full story you only need two
measurements and then you can can calculate the third as the airframe load
plus the charging current will equal the alternator output. You really only
need one measurement (alternator output) as airframe loads are are easy to
characterize and are typically stable.
Prior to first flight you should have measured the actual current draw of
each of the loads in your aircraft. In my case I removed the fuse or opened
the breaker and then shorted across it with a portable (Fluke) ammeter. The
measured values were entered into a spreadsheet that listed all the loads.
Loads were characterized by type (normal, emergency, intermittent etc.) and
could then be sorted to produce an accurate prediction of the airframe loads
in various configurations. All this should be done on the ground.
Another thing to consider when flying with a dead or week battery, the HPU
(hydraulic power unit) typically has a maximum current draw (especially on 12
volt systems) greater than the output of the alternator. In order for the HPU
to reach its design pressure the battery MUST supply part of the current. An
electric motor's torque is directly proportional to its supplied current. As
the hydraulic pressure in the accumulator rises so does the torque and
therefore the current. The breaker is sized for the maximum motor current plus
a small margin. So, what happens if the HPU needs more current than is
available? Simple, it stalls.
Take the theoretical (numbers here do NOT reflect any actual configuration
and are not represented as typical) case where you have a HPU that needs 70
amps to make enough pressure to turn off the pressure switch. The HPU is wired
to a 75 amp breaker and the airframe has a 60 amp alternator with a 12 amp
typical airframe load. For a few seconds of HPU operation the battery must
draw 27 amps from the battery. If that current is not available because the
battery is dead then the HPU will stall AND CONTINUE TO DRAW CURRENT. If the
motor armature is not turning then the buss voltage will brown down to the
product of the of the available current times the motor's DC impedance (E= I x
R). The motor armature is now rapidly heating so its DC impedance is
increasing and the buss voltage is rising (the current remains constant).
Since power is the product of voltage and current, as the armature heats and
the voltage across it rises, the power going into it, heating it, increases
causing the impedance to increase causing more power ......... until the HPU
motor gets "smoking hot", literally. So now we have red hot copper and iron
suspended over a gallon of diesel fuel (AKA hydraulic oil). Did I mention that
in the vicinity the oil is at high pressure in thin walled aluminum tubing?
While the HPU motor is performing its rendition of the China Syndrome what
does the pilot see? The buss voltage will momentarily brown down (maybe 8
volts) but most of the airframe loads are designed to continue to operate at
this voltage and then rise back up to near normal levels. The ammeter will
show maximum load and the HPU breaker will NOT trip as the problem is too
little current not too much. OK, so no hydraulic power, no big deal, I can do
a no flap landing and use the emergency pump. But what about the poor
alternator? It has been producing maximum output for most of the flight and it
wants a "break". If the alternator gives up you no longer have power.
So here is the situation. You will need to make an emergency landing with no
flaps, no gear lights, no radio, no engine instruments, no position lights or
strobes, no fuel gages, no fuel boost pump, no transponder, no trim control,
no GPS AND a possible fire in the tail. No problems, Mate. You were not
distracted by the emergency and you know your position, you have the Emergency
Landing, Fire and No Radio checklists out, you have the correct paper chart
handy, you have identified the nearest airport and am sure you won't trespass
on restricted airspace so some nice National Guard pilot won't be ordered to
shoot you down. Easy......
Now someone wants to stand up and tell me that taking off with a dead battery
is OK because of some nuanced reading of the FARs??? Give me a F*%&#$G break!
Airliners have exploded in mid flight from situations similar to the one
described. The EnginAir V8 company was destroyed and people were killed
because someone elected to fly with a problem with the primary electrical
system. The list goes on and on because there is no limit to human ignorance.
It can be said for EVERY pilot that there are elements of the miracle of
flight that they do not fully understand. When presented with an abnormal
situation it IS an unsafe condition because the pilot cannot know all the
consequences. Did you know about the potential HPU scenario presented above
(rhetorical interrogative, not a solicitation for response)? Do you know
about ALL the possible system interactions that may lead to unsafe conditions.
The accident investigators will wonder why you continued the flight with such
an obvious problem.
Ignorance does not equal safety. When traveling at almost half the speed of
sound five miles up, what you don't know will kill you if given the chance.
Play it safe.
Regards
Brent Regan
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