Fuel Systems

Volatility

the measurement of a liquid’s ability to convert into a vaporous state, effects engne starting, range, and safety.

Volatility is NOT

a volatility rating

Flash Point

lowest temperature of a fuel that would ignite with a momentary application of a flame, is an index for a fuel’s safety and storage

As volatility increases

flash point decreases

JP-4

highly volatile, low flash point, easier starting, higher tendency to vapor lock, shorter range

JP-5

primary jet fuel for the navy, thermally stable and meets navy safety requirements due to low volatility

JP-8

similar to JP-5, used by Air Force due to lower flash point

AV Gas

has the same flashpoint as gasoline, green in color

AVGAS 100LL

low lead, blue in color

Fuel Tank

The starting point for fuel, can be fueled by gravity, pressure, or mid air

Boost Pump

ensures adequate supply of fuel to fuel pump; can be used to transfer fuel to other tank for weight and balance; supply fuel pressure to start engine and supplies fuel to primer; prevents aeration of fuel supply

Emergency Shutoff

used in emergency to shut off fuel flow to engine

Fuel Pressure Gauge

receives signals from pressure sensor at the boost pump to indicate the absence of fuel or a failed fuel pump

Low Pressure Fuel Filter

strains impurities from fuel and absorbs water; susceptible to clogging and needs a bypass valve to ensure fuel flow

Engine Driven Pump

High pressure pump designed to deliver fuel to FCU, provides fuel in excess to meet engine and afterburner requirements

Fuel Control Unit (FCU)

brain of the fuel system, consists of fuel metering and computing systems, sends measured fuel to satisfy fuel flow requirements for each stage of flight

What factors does the computing system (FCU) take into account

Compressor Inlet Temperature, RPMs, Turbine Temperature, PCL Position

Normal (Automatic) FCU Operation

provides proper fuel flow for every operating condition and PCL input, under this, engine should remain stable within normal operating ranges

Manual/Emergency FCU Operation

bypasses FCU if it fails; lacks acceleration limiting and RPM governing capabilities; PCL controls fuel flow, requires close monitoring by pilot

Fuel Flow Gauge

measures fuel flow rate out of FCU and indicates fuel consumption in lb/hr

Fuel Manifolds

delivers fuel to engine burner through a series of nozzles; consists of primary and secondary lines

Primary Manifold Lines

have smaller diameter tubes to allow high pressure during starting and high altitude idle conditions

Secondary Manifold Lines

supplies fuel as RPM raises fuel pressure

Pressurizing and Dump Valve

serves to drain the combustion manifold upon engine shutdown and adjust fuel flow during startup

What can occur is residual fuel remains in the combustion manifold

engine fire

Rated Thrust

based on turbine temperature

Normal Rated Thrust (NRT)

thrust produced at max continous turbine temperature with NO TIME LIMIT; serves for cruise speed

Military Rated Thrust (MRT)

thrust produced at max turbine temperature with a time limit, can serve for takeoff

Combat Rated Thrust (CRT)

thrust produced during afterburner operation, NOT based on turbine temperature