View Full Version : A320 Rat To Start The Apu
10th Feb 2010, 17:07
Just an idle moments thought.
When in emrg elec config you don't start the APU in order to save battery life ( 3 mins per APU start ). Why not have a separate ' RAT' like item on the APU, with the sole job of starting the APU independently of batteries?
Would it not make those long T16 nights a little more warm and fuzzy?
10th Feb 2010, 17:39
RAT generates electrical power hardly enough for AC ESS and DC ESS busses (not SHED busses), and BLUE HYD SYS. Battery is meant as the last mean of supplying power to the a/c while in EMER ELEC CONFIG so had better not to be used for an APU start attempt. A second RAT surely can't be the solution, otherwise it will be necessary to install another EMER GEN and its own GCU, thus significantly increasing the aircraft weight, also keepin' in mind that a RAT can stall and must be in the right place ... just my thought ! ;)
10th Feb 2010, 19:58
The thing is, how useful would the APU actually be in the emergency electrical configuration?
Since one engine generator can drive the whole network, it seems likely that to end up in the emergency configuration the AC buses would be so damaged that the APU generator would be unlikely to latch anyway. Worth gambling away 3 mins of battery time?
Alternatively, if you established the configuration due to smoke, then you wouldn't need the APU generator.
10th Feb 2010, 20:18
Fly with no fear along the T16, you lucky man, when the god of wind so dictates.
Start the APU if you had a double GEN fail. Don't, if it was double AC BUS fail (it is useless).
Losing HOT BUS 1 and 2 is not that bad. With the RAT you have all the essential things.
10th Feb 2010, 21:16
it seems likely that to end up in the emergency configuration the AC buses would be so damaged that the APU generator would be unlikely to latch anyway
How useful the APU in EMER ELEC CONFIG ? Would be useful at the point that we can go out of EMER ELEC CONFIG ...
Dual GEN/IDG failure or TR1 Failure + GEN2 Failure and vice-versa ... why should AC buses be that damaged ? I mean APU GEN can yet be connected and supply the whole network here ... :confused:
10th Feb 2010, 21:23
1. With TR1 Failure + GEN2 (or vice versa) failure you are not in ELEC EMER CONFIG anyway. Sure, start the APU. All buses are powered.
2. RAT (EMER GEN) powers the essentials, shed buses included.
10th Feb 2010, 21:38
I do know we're not in ELEC EMER CONF in the situation previously mentioned ... here's the matter: couldn't quite understand why should the AC busses be so damaged for an APU start to be useful ...
Yep, RAT powers AC ESS + DC ESS + SHED BUSSES, was wrong here ... just mixed up with BAT's power only.
10th Feb 2010, 21:51
aristoclis - I was thinking of it this way:
Gen 1 and 2 failure... if it didn't happen simultaneously, then the APU will probably already be running following the first failure. If GEN 1 failed and then GEN 2 failed almost immediately afterwards, an APU start is still possible at the expense of battery life.
AC BUS 1 + 2 failure... well, there's no available AC bus to connect the APU to, so why start it? :)
10th Feb 2010, 22:00
Hi David Horn, are you talkin' to me or airstoclis ?
anyway now I got it ... ;)
10th Feb 2010, 22:29
If the failure is AC BUS 1+2, it is obvious tha APU is useless. But in the case that both GEN fail simoultaneously or with short intervall, consider if the failure was caused by something that would cause the APU GEN to fail also, as soon as it powers the AC buses. So be sure not to drain your batteries and end up with a failed APU GEN...
10th Feb 2010, 22:47
What may be usefull is a apu rat that direct drives the gearbox as the electric starter does, no battery usage, just a complicated instalation. A gearbox mounted rat that drops out of the APU bay.
11th Feb 2010, 07:26
You've got it, thats what I was getting at. If you can get the apu gen on line without the use of the batteries, you wouldn't be in emerg elec config.
11th Feb 2010, 07:52
Would anyone try to install a RAT near the APU / tail cone ? surely either it would stall out regardless of speed or sense reverse aerodynamic flow, depending on how will it be oriented. Not feasible ...
11th Feb 2010, 09:28
It would stop a tail scrape!
11th Feb 2010, 13:26
In case of dual GEN fail, start APU. All network supplied, except galley.
In case of dual AC BUS fault, forget about the network. Only essential buses supplied by the RAT EMER GEN. What do you want the APU for. To supply the DC BAT and the HOT buses?
Losing HOT BUS 1 and 2 is not that bad, is it?
with the RAT we have all we need
14th Feb 2010, 05:53
Surely 2 different mod states to consider here.
For the early 320's follow the checklist which starts with an attempted reset of both generators, if that is unsuccessful then you are stuck in emergency electrical config and an APU start is not reccommended.
For later 320's and all (?) 321's an enhanced RAT is installed, in this case an APU start is allowed after 45secs have elapsed since the loss of both eng generators.
So the enhanced RAT gives the cozy feeling requested on T16, change was somewhare around s/n 1000 for the 320 I think.//