Engine Indications and Condition Monitoring

Engine indications tell the crew what each gas turbine engine is doing and whether it is doing it within limits. The ATPL texts divide them into two groups. Performance indicators show how much thrust the engine is producing: N1 or engine pressure ratio (EPR). Condition indicators show whether it is healthy while it does so: exhaust gas temperature (EGT), compressor speed, oil pressure and temperature, and vibration.

On a glass flight deck the primary parameters stay in view all the time. On the A320 the upper ECAM display permanently shows N1 (or EPR on V2500 engines), EGT, N2 and fuel flow, while oil data and vibration are on the ENG system page; Boeing's EICAS puts N1 and EGT on the upper display and the secondary engine data on the lower one. The same numbers, recorded flight after flight, feed condition monitoring, which aims to find a deteriorating engine long before it fails.

Spool speeds: N1, N2 and N3

RPM on a gas turbine is shown as a percentage of a reference speed rather than in revolutions per minute. N1 is the speed of the low-pressure spool: the fan, the low-pressure compressor and the low-pressure turbine that drives them. N2 is the speed of the high-pressure spool, the core compressor and its turbine. The starter turns N2, and N2 drives the accessory gearbox. On a three-spool engine a third speed is added: N1 remains the LP spool, N2 becomes the intermediate spool and N3 the high-pressure spool.

On CFM and GE engines N1 is the thrust-setting parameter; the 737 NG's EEC, for example, sets thrust by N1 in both of its control modes. The relationship is far from linear: on a high-bypass engine about 25% N1 gives only about 5% of take-off thrust, so large rpm changes make small thrust changes at low power and small rpm changes make large thrust changes near maximum.

Spool speeds carry structural limits. On the CFM56-5B the N1 red line is 104% and the N2 indication turns red above 105%. An exceedance leaves a record: after an N1 exceedance on the A320 a red mark shows the highest value reached and is cleared only by a new engine start on the ground or a maintenance action, while the 737 frames the N1 or N2 readout in a red box after shutdown if an in-flight exceedance occurred.

Engine pressure ratio

Engine pressure ratio (EPR) is the ratio of the pressure at the turbine discharge or in the jet pipe to the pressure at the engine intake. The intake pressure is sensed by a probe at the engine face, often called the P1 probe in the ATPL texts, which number engine stations from the intake. Because EPR follows thrust closely, many Pratt & Whitney and Rolls-Royce engines use it as their thrust-setting parameter. On the A320 the IAE V2500-A5 is set by EPR, with N1 as a secondary indication, while the CFM56-5B is set by N1.

Older engines used a P7 gauge, which showed jet pipe pressure alone, in inHg, psi or per cent of maximum thrust. Some large turbofans use integrated EPR (IEPR), which weights the fan outlet pressure together with the turbine discharge pressure before comparing them with the inlet pressure, because most of a turbofan's thrust comes from the fan.

EPR has one well-known quirk. As the aircraft accelerates on the take-off roll, ram effect raises the intake pressure while the jet pipe pressure hardly changes, so the indicated EPR falls although the thrust has not. A pilot who kept adding thrust to restore the figure could exceed the N1, N2 or EGT limits. The ATPL texts therefore say to set take-off EPR before about 60 kt and not to increase it after that speed except in an emergency.

Fuel flow indication

A fuel flowmeter in the high-pressure fuel line to the spray nozzles measures the fuel delivered to the engine. A common design has a helical vane impeller carrying a magnet, which spins in the fuel flow and induces a signal in a pick-off coil at a frequency proportional to the volume flow; correcting for fuel temperature gives mass flow in kg/h or lb/h. On the 737 NG the fuel flow readout is shown in kilograms per hour × 1000.

Integrating the flow over time gives fuel used, which the A320 resets when the ENG MASTER switch is set ON on the ground. Comparing fuel used with fuel remaining is the crew's main check for a fuel leak (see in-flight fuel management). At start, the 737 indication lags the actual flow by about two seconds, so the EGT may rise before any fuel flow is shown.

EGT, TGT and ITT

The gas temperature at turbine entry is the real limit of the engine, but it is too hot to measure in service (see combustion chambers and turbines). Engines measure the gas further aft instead, with thermocouples of chromel and alumel, alloys that withstand very high temperatures and give a usable voltage per degree. Several probes are spaced round the gas stream and wired in parallel, so the indication is their average and one failed probe causes only a slight drop. The thermocouples generate their own voltage, so the indication needs no power supply.

The name depends on where the probes sit. Exhaust gas temperature (EGT) is the general term and the one used on the A320 and 737. Jet pipe temperature (JPT) is taken in the jet pipe of older engines. Turbine gas temperature (TGT) is measured in the turbine, and inter-turbine temperature (ITT) between the HP and LP turbines, as on the Embraer E-Jets. Because the station differs, so does the limit: the CFM56-5B EGT red line is 950 °C, the V2500-A5's 635 °C.

View full schematic

EGT reacts to anything that leaves less air to cool the same fuel flow. It rises when bleed air or engine anti-ice is taken off the compressor, during a compressor stall, together with vibration, and in a hung start, where it is high for the low rpm.

Redlines and EGT margin

Engine displays follow a colour code: green for the normal range, amber for caution, and red for a maximum or minimum limit, the redline. On the CFM56-5B the EGT red line is 950 °C, the limit for take-off and go-around thrust, for 5 minutes with all engines operating or 10 minutes with one inoperative. An amber mark at 915 °C is the maximum continuous limit, and during a start the amber mark moves to 725 °C, the start limit. Above 950 °C the value pulses red and a red mark records the maximum reached.

The 737 NG shows the maximum take-off EGT as its EGT redline; the lower end of an amber band is the maximum continuous limit, and the amber colour change is inhibited for up to five minutes during take-off or go-around. An EGT start limit redline is shown during ground starts, and some in-flight starts, until the engine reaches stabilised idle at about 59% N2.

The engine control does not generally protect the EGT limit. The A320 FADEC protects N1 and N2 at all times but EGT only during automatic ground starts, and the 737 EEC gives N1 and N2 redline protection but no EGT redline protection, its hot start and wet start protection working only during ground starts. Watching EGT is the crew's job (see FADEC and engine fuel control).

EGT margin is the difference between the EGT the engine reaches and its limit. It shrinks as the engine ages, because a worn compressor or turbine needs more fuel, and runs hotter, for the same thrust. A flat-rated engine meets its EGT limit on hot days, so the margin decides how much thrust a tired engine can still deliver. Reduced take-off thrust lowers EGT and slows the loss of margin.

Vibration monitoring

Engine vibration monitoring senses vibration with a piezoelectric crystal or a magnet and coil mounted on the engine casing. The signal is filtered to keep only the frequencies that indicate damage and is displayed in relative units. Modern systems monitor each rotating assembly separately, so the source can be identified.

On the A320, accelerometers on the engine casing give VIB N1 and VIB N2 on the ECAM ENG page; the readings pulse amber above 6 units for N1 and 4.3 for N2. High N1 vibration suggests fan imbalance, foreign object damage or blade damage; high N2 vibration points to the core. The 737's airborne vibration monitoring (AVM) system displays the synchronous vibration of the LP and HP rotors on the secondary engine display and is also used on the ground to balance the LP rotor. If the 737's EEC is unpowered, N1, N2, oil quantity and vibration are still displayed directly from their sensors.

Engine health and trend monitoring

Engine health monitoring (EHM) turns indications into maintenance decisions. The engine control stores parameters such as vibration in its memory, maintenance downloads them, and many aircraft send them in flight by datalink (see ACARS and aircraft datalink). Trend monitoring compares readings taken in stable conditions with the engine's own history and with the fleet. A gradual rise in EGT, fuel flow or vibration shows progressive deterioration; a sudden step points to an event such as foreign object damage, blade damage or a failing bearing.

Borescope inspection

When the numbers point inside the engine, a borescope inspection looks there. A borescope is an optical probe, rigid or flexible, inserted through inspection ports to view compressor blades, the combustion chamber, nozzle guide vanes and turbine blades without taking the engine apart. It is used after events that may have caused damage, such as a vibration spike, a bird strike or an over-temperature, since turbine blades may have been damaged.

Library articles are written for study and exam preparation. They do not replace your aircraft's approved documentation, your operator's procedures or the regulations themselves.