Hydraulic System (i.e. Brakes & Cowl Flaps) B-17 Technical Session for Flight Engineers 7/8/2017 (with post meeting revisions 7/9/2017) The B-17G (specifically our Texas Raiders, TR) has a hydraulic system that services only TWO functions the brakes and the cowl flaps. Nothing else on the airplane works off this system. The hydraulic system consists of three major components: 1. The hydraulic power unit this generates and maintains a source of high pressure hydraulic fluid. 2. The main landing gear brakes two systems; LEFT & RIGHT brakes. 3. The cowl flap activation four systems; one for each engine. The hydraulic power unit consists of a high pressure hydraulic oil Accumulator, a pressure make up pump, an on-off switch that operates the pump, and a reservoir for standby supply of hydraulic oil. (There is also an in-line oil filter, a pressure relief valve, a hand operated standby pump and a pressure gage in the works). The heart of the system is the Accumulator. The pressure in the Accumulator is what drives the brakes and cowl flaps to function. Everything else in the hydraulic power unit is there to maintain or monitor the Accumulator pressure. The Accumulator has a floating piston inside with nitrogen on one side and the hydraulic oil on the other. Pumping up the Accumulator with oil squeezes the nitrogen and that becomes the stored energy which makes it all work. The nitrogen has a pre-charge of 350 psi and increases as the pumped oil pressure exceeds the 350 PSI. The pump is driven by a DC motor and works off TR s electrical system. The pump on-off switch is pilot controlled and has three positions; MANUAL OFF AUTOMATIC. MAUNUAL works the pump regardless of Accumulator pressure. OFF is off. AUTOMATIC turns the pump ON at any time the Accumulator pressure is below 600 PSI, and OFF when the pressure reaches 800 PSI. Accumulator pressure below about 550 PSI turns ON a warning light on the instrument panel. Accumulator pressure above about 875 PSI opens the relief valve, which then closes at about 850 PSI dumping the oil back to the reservoir. The oil filter has an internal bypass valve so if the filter gets clogged the oil still flows. The pressure gage reads the hydraulic oil pressure in the Accumulator. The hand operated pump is a backup to the motor driven pump (co-pilot operated). The reservoir holds supply hydraulic oil for the pump - at zero pressure. The main landing gear brakes consists of the pilot & co-pilot s toe brakes (actually hydraulic metering valves), de-boost valve and the wheel brake assembly. The LEFT & RIGHT brake systems are identical but operate independently, with the only common part being the pressure feed from the hydraulic power unit. The toe brakes meter the pressure to the brakes the more the brake is pressed the more the braking action. The
de-boost valve does TWO primary functions ONE it reduces the hydraulic feed pressure by a third (600 PSI becomes 200 PSI). SECOND it unloads the brakes by pushing the hydraulic oil back to the reservoir once the pressure is relieved - there is a big spring inside the valve acting on a piston that does that. NOTE: The reason the pressure is reduced is to trade a small-volume-of-oil-at-a-high-pressure for large-volume-of-oilat-a-low-pressure. This is what gives the brakes the sensitivity needed for steering the aircraft. Also there is some clever engineering inside that allows the system to be self bleeding of any trapped air and also make up for leaked hydraulic oil in the brake assembly. The brake assembly (two for each wheel four total) has an expander tube (a flat inner tube looking thing) that expands under hydraulic pressure and pushes brake pucks into the brake drum. Upon activation, high pressure hydraulic oil from the Accumulator cycles down to the de-boost valve, then once the toe brakes are released, the same oil (now low pressure) then cycles back to the reservoir.. The hydraulic oil below the de-boost valve stays put, only cycling from the de-boost valve to the expander tube and back to the valve again. The cowl flap activation consists of a control valve and a hydraulic cylinder four times. All four systems are systems are identical but operate independently, with the only common part being the pressure feed from the hydraulic power unit. Each of the four control valves has three positions; OPEN LOCKED CLOSE. OPEN and CLOSE do just that for the cowl flaps. The amount of OPEN can (and is) varied by the pilot to control engine temperature. The LOCKED position freezes the cowl flaps at the last position set by the pilot. A hydraulic cylinder actually moves the cowl flaps all the cowl flaps on an engine are connected by mechanical linkage to the one cylinder. The cylinder is double acting works both ways. Upon activation, hydraulic oil from the Accumulator cycles down to one side of the hydraulic cylinder while the non-pressured side of the cylinder cycles oil back to the reservoir. FE Advisories: 1. Hydraulic oil is pumped from the reservoir to the Accumulator to create the hydraulic pressure. That means the oil level in the reservoir is the lowest when the hydraulic pressure is the highest, - and vice/versa. The time to check the level in the reservoir (with the dipstick) is when the hydraulic pressure is at the maximum operating pressure (about 800 PSI). 2. The hydraulic pump makes up hydraulic pressure consumed (depleted) during operation of the airplane. Pressure is consumed a lot during taxi, takeoff and landings AND almost not at all during cruise. So you should hear the pump running a lot when the plane is on the ground AND it should be almost dead silent while in flight. Anything other that that, then there is something wrong check it out! 3. In a perfect world, the hydraulic system being self contained should never need oil added but leaks do happen. There is a 1 gallon can of hydraulic oil stationed next to the reservoir should adder oil be needed. There is more in the radio room floor storage.
4. Since the pump operates automatically on hydraulic pressure, the level of hydraulic fluid in the reservoir should always stay about the same (i.e. oil returning back to the reservoir is quickly pumped back to the Accumulator to maintain pressure). A falling level in the reservoir is indication of a leak somewhere. 5. The dipstick has several small holes in it to help read the level. It is easier to read if the dipstick is held up to the light coming through the cockpit windows. The holes below the level wet line will appear red from the hydraulic fluid. 6. The hydraulic power unit has a RED valve on it. Opening this valve dumps all of the systems hydraulic pressure back to the reservoir. This is used for maintenance only and should never be opened during any flight situation. 7. The 350 PSI pre-charge on the Accumulator nitrogen is essential. Watch the hydraulic pressure during pre-flight working of the brakes and the cowl flaps (with the pump switch set at AUTOMATIC). If that action causes the hydraulic pressure to drop dramatically, the 350 PSI nitrogen pre-charge has been lost and it is a NO-GO for take off. Should that happen, the system is working only on the pump and that alone is not adequate. 8. Hydraulic pressure must be above 600 PSI before engine start (to lock the brakes) and especially before taxi and take-off. Check the gage. Low hydraulic pressure is a NO-GO situation. 9. Hydraulic pressure needs to be above 600 PSI before landing. Check the gage. If necessary, add hydraulic oil, operate the pump on MANUAL, use the hand operated standby pump, whatever is required, to attain the needed pressure. 10. The de-boost valve has several small vent holes on the side of the valve. There is a piston inside the valve which has O rings. Leakage about these O rings will exit out these small holes. Some leakage there is normal large leakage is an O ring problem. 11. The expander tubes can split and leak. Hydraulic oil will accumulate at the lowest point on the brake assembly when this happens. During pre-flight and after every landing the FE needs to check the expander tubes for signs of leaking. A leaking expander tube is a NO-GO for flight. (Usually when this happens, the entire brake assembly is replaced rather than just the expander tube a spare is kept in the airplane) Note that when a brake assembly is replaced there is no need to bleed the air from the new expander tube the system is designed to self bleed. To affect the self bleeding feature, the pressure to the new tube needs to be cycled several times. Also the connecting hose couplings are self sealing, so there should be a minimal loss of hydraulic fluid during the replacement). Replacing a brake assembly requires jacking up one side of the airplane to work on a wheel. The big tri-pod jack acting under the wing is best for this. If that jack is not available, a bottle jack under the landing gear strut can be used. When using the bottle jack, the tail of the airplane needs to be tied laterally (tie to the tarmac D rings, or an automobile, or something) to prevent a wind gust from potentially twisting the airplane off the bottle jack).
FE Operational Awareness summary regarding the Hydraulic System Observe the following and all should be OK: 1. Check the hydraulic pressure. If the pressure is low or NO pressure, then it s NO, NO, NO. NO engine start. NO taxi. NO take off. 2. Note the power unit action. On the ground it should be busy and noisy (pump on/off, pressure changes, oil level movement). In flight it should do nothing. 3. Watch the reservoir oil level (Check at engine start, run-up, at cruise, in-range and after landing). If the oil level stays the same, then there are no significant leaks.