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A300Capt · January 23rd, 2003, 17:00

[ QUOTE ]
Just out of interst if the battery had died would they lost hydraulic pressure i.e the ability to control the airplane? Or would it have just been cockpit instruments?

[/ QUOTE ]

The B757 has 3 hyd systems. The left and right systems have one engine driven pump and one electric pump while the smaller, center system is controlled only by 2 AC electric pumps. Obviously, they would have lost the center system..no big deal.

The left and right engine pumps would have continued to operate as those pumps are "fail safe" meaning they require electrical power to depressurize or turn off.

The problem is that if you are down to just the battery power, and keep in mind, you have to lose multiple layers of electrical backups before getting there, you're then down to only 4 small buses (Batt, hot batt, standby AC/standby DC). All these require at least battery power at that point to operate.

If you now lose the battery (after approx 30 minutes if you're lucky), you lose all power to the last 4 buses. The eng hyd pumps will continue to operate as long as the engines are running and thus primary flight controls are powered. The engines are now suction feeding (engine fuel pumps are inop) and the FADEC's EECs are being powered by dedicated permanent alternators which are completely independent of normal ship's power.

The crew would have no instruments/radios to guide them if the battery depleted. Not even the emergency standby instruments. If you could get the gear/ flaps down you would have no indications of their position.

BTW, many domestic B757's don't have HDGs (hyd driven generators) that automatically kick in, assuming you have hyd power, in the event of a complete AC power source failure.

Bottom line...had the crew not been able to re-start the APU and get it's generator online and continued flying around on a slowly dying battery, they would have eventually lost everything. Also remember that everytime they attempted to start the APU it required more precious battery power which they couldn't afford to give up at that point.

Probably more info than you wanted.

January 23rd, 2003, 17:00	#4
A300Capt Senior Member Join Date: Oct 2000 Location: Louisville, Kentucky Posts: 707	Re: Not suppose to happen.... [ QUOTE ] Just out of interst if the battery had died would they lost hydraulic pressure i.e the ability to control the airplane? Or would it have just been cockpit instruments? [/ QUOTE ] The B757 has 3 hyd systems. The left and right systems have one engine driven pump and one electric pump while the smaller, center system is controlled only by 2 AC electric pumps. Obviously, they would have lost the center system..no big deal. The left and right engine pumps would have continued to operate as those pumps are "fail safe" meaning they require electrical power to depressurize or turn off. The problem is that if you are down to just the battery power, and keep in mind, you have to lose multiple layers of electrical backups before getting there, you're then down to only 4 small buses (Batt, hot batt, standby AC/standby DC). All these require at least battery power at that point to operate. If you now lose the battery (after approx 30 minutes if you're lucky), you lose all power to the last 4 buses. The eng hyd pumps will continue to operate as long as the engines are running and thus primary flight controls are powered. The engines are now suction feeding (engine fuel pumps are inop) and the FADEC's EECs are being powered by dedicated permanent alternators which are completely independent of normal ship's power. The crew would have no instruments/radios to guide them if the battery depleted. Not even the emergency standby instruments. If you could get the gear/ flaps down you would have no indications of their position. BTW, many domestic B757's don't have HDGs (hyd driven generators) that automatically kick in, assuming you have hyd power, in the event of a complete AC power source failure. Bottom line...had the crew not been able to re-start the APU and get it's generator online and continued flying around on a slowly dying battery, they would have eventually lost everything. Also remember that everytime they attempted to start the APU it required more precious battery power which they couldn't afford to give up at that point. Probably more info than you wanted.