# Are engines fed differently in reduced gravity aircraft?

Reduced gravity aircraft are basically airliners without passengers seats. As airliners, those aircraft were not designed to fly for a long time in zero-G. Specifically engines were not designed to be fed with fuel for several tenth of seconds in reduced gravity conditions. I'm aware that aerobatic aircraft are equipped with devices to feed the engine in negative-G conditions for a limited amount of time, but what about the reduced gravity aircraft?

EDIT: aerobatics aircraft are equipped with header tanks and/or flop tube able to get fuel in tank even if flying inverted (e.g. flop tube's end goes to the bottom of the tank, bottom defined in Earth frame of reference, whatever the attitude of the airplane). This design allow pipes to always be inside the liquid fuel of the fuel tank, and thus pumps can do there job (pump liquid). But when in zero G, liquid is no longer attracted toward one side of the tank (either the bottom or other side) and thus pipes may be fed with a mix of air (or whatever gas or vapor that is inside the fuel tank) and liquid fuel. Pressurized tanks do not change this. Yet, engine must be fed in fuel for several tenth of seconds in those condition, and I doubt fuel contained in pipes can last more than few seconds. Devices to fed a reduced gravity aircraft's engine should be different from the one in aerobatics in at least two way:

• aerobatics are designed to operate in inverted flight (not zero-G) and are in zero-G condition for a small amount of time (less than a couple of aerobatic maneuvers before being again in normal or inverted flight)
• reduced gravity aircraft are airliners (may be modified, but how?)
• The two questions are indeed duplicates, but I don't understand why the (7 month-)older is marked a a duplicate of the newer. Chronologically speaking, it should be the opposite, as explained here – Manu H Apr 7 '16 at 9:14
• The newer question is more specific and seems to have a more authoritative answer. As explained on meta: The general rule is to keep the question with the best collection of answers – fooot Apr 7 '16 at 16:00
• @fooot the part of the newer question edited less than 3 hours ago makes both question not duplicates (and explicit why it is not) – Manu H Apr 7 '16 at 16:08
• @ManuH (or anybody else who can confirm the intent and justify the edit) says "tenth of seconds" a couple of times. In the last paragraph you follow it up with "more than a few seconds". Should this say "tens of seconds" (meaning some number more than 10, but less than 100) or "tenths of a second" (meaning some number less than 1)? – T.J.L. Apr 8 '16 at 18:49

Zero 'g' aircraft are basically commercial aircraft.

In general, it is mainly the fluid systems like the fuel and hydraulic systems in the aircraft which are affected by the zero gravity conditions.

In the case of jet engines, the fuel is transferred by pumps operating under pressure and the fuel injection is also under pressure in the combustion chamber. Consequently, the engine fuel system does not need any modification. In a few aircraft, the fuel tanks themselves are pressurized (like C-5) but this is for safety and wouldn't affect operations.

The engine oil and lubrication system is also usually pressurized and can operate briefly under reduced gravity conditions. Work done by NASA indicates that aircraft can be used in zero gravity conditions without any major modifications to systems.

The zero 'g' aircraft don't operate in weightless environment for more than 20-30 seconds and the engine systems should be able to cope during that time. The main concern appears to be the training levels of the pilot.

• Can you elaborate on the concern about pilot training levels? – curious_cat Aug 19 '15 at 7:43
• OK for the part between the pumps and the engines, but what about the part befor the pumps? Pipes going from the fuel tank to the pump should begin at the bottom of the tanks, in zero-g conditions you cannot garanty fuel is at the bottom of the tank. – Manu H Aug 19 '15 at 8:36
• The document says the pipes themselves contain enough fuel for 50s, which is more than sufficient. I guess there is some gravity-based gas/liquid separation in each engine and the fuel pump is much faster than what the engine needs. So bubbles may arrive after the zero-g period and are vented. Also, I guess taking fuel from a full tank reduces the risk of bubbles. But as said, just a guess, no definite answer. – sweber Aug 19 '15 at 10:51
• There have been several references here at Aviation.SE that fuel is often contained in a collapsible rubber bladder within the wing tank location to prevent the build up of fuel vapor. If that's the case, then the pump sucking fuel out of the bladder would cause the bladder to collapse, no matter the amount of gravity, and that would keep the feed line in constant contact with the fuel contained within. – FreeMan Aug 19 '15 at 15:29
• How would the hydraulic system be affected by zero gravity? Please explain. – Peter Kämpf Apr 8 '16 at 6:12