Turbine Pilot

Measuring Power

December 1, 2001

Performance gauges for jet engines

Turbine performance gauges are one of the items that new jet pilots have not experienced in their previous flying. They're of vital importance in setting and measuring power, and they are different from piston gauges — but not any harder to understand. Engine gauges for jets typically have both an analog dial indicator (or, in the case of the Cessna Citation, a vertical tape) and a digital readout. Before we examine the different gauges, let's break them into two groups — power-setting gauges and engine-monitoring gauges.

Power-setting gauges

The two main types of power-setting gauges for jet engines are the EPR gauge and the N 1 tachometer. The EPR (engine pressure ratio) gauge indicates a ratio calculated from the turbine discharge pressure divided by compressor inlet pressure. Engines that use EPR gauges are known as pressure-rated engines and typical EPR indications are between 1.5 and 3.0. EPR gauges can also be used for engine monitoring in a speed-rated engine.

Speed-rated engines use an N 1 tachometer gauge to set power. This gauge is usually calibrated in percent of maximum rpm, but it may also indicate actual revolutions per minute. Under certain ambient conditions, it is permissible to have an N 1 reading above 100 percent. This doesn't seem to make sense, but it only means that the indicated engine rpm is more than the rpm required to achieve full-rated power under the conditions at which the certification rating was achieved. N 1 speed is the speed of the low-pressure compressor, or fan, in a twin-spool engine such as the Pratt & Whitney JT15D (see " Turbine Pilot: Jet Engine Basics," September Pilot). (The speed of the high-pressure compressor or its associated turbine is shown on an N 2 gauge.)

Some pilots use fuel flow gauges as power-setting aids. These gauges are far more accurate on turbine aircraft than on piston aircraft. They're calibrated in pounds per hour since a jet engine's fuel burn is calibrated by weight, not volume as in piston engines. Fuel, when burned at the optimum fuel-to-air ratio, provides a known amount of power per pound, making it possible to gauge power developed by fuel consumed. All fuel in a jet engine is used for power generation while some fuel in a piston engine is used for cooling.

Performance gauges

Other gauges on the jet panel are used primarily to monitor and limit the performance of a jet engine, and principal among these are the temperature gauges. These are labeled turbine inlet temperature (TIT), exhaust gas temperature (EGT), or interturbine temperature (ITT) gauges depending upon where the temperature probes are located in the engine. As in virtually any engine, heat — the graveyard of energy — is the enemy of jet engines. Jet engines always have enough air available to burn the fuel. What is sometimes lacking is sufficient air to cool the engine, and temperature is one of the major limiting parameters for these engines. Most engines have !imits on maximum temperature for each phase of operation, including start, takeoff, climb, maximum continuous power, and cruise power. Temperature limits during reduced power operations are usually not a concern.

Another engine parameter used as a limit is the speed of the N 2, or high-pressure, turbine. This is the turbine that supplies power to the high-pressure compressor. Turbine blade tip speeds are a major limiting factor for jet engines, and the speed at the tips depends not only upon the revolutions per minute of the turbine but also on the diameter of the turbine disk. Transonic and supersonic tip speeds would cause shock waves to form and disrupt the airflow through the engine. N 2 limits are expressed in percent of maximum rpm — and rarely in actual rpm — and are usually kept several percentage points below 100 percent. Turbine rpm is much higher than propeller rpm in piston engines, and in small turbine engines rpm can be in the tens of thousands. Although turbine rpm is sometimes used as a power gauge, it is more accurately called a limitation gauge since the power developed for any given rpm is also dependent upon ambient temperatures and pressures.

Other gauges in a jet aircraft are similar to those found on other aircraft. Oil temperature and pressure, hydraulic pressure, amperage, voltage, and cabin pressurization indicators are all found in various combinations on the panel.

Setting power

Unlike most nonturbine aircraft, takeoff calculations in a jet include calculation sf the power setting for ambient conditions. Ambient temperature and takeoff field elevation are used to calculate takeoff power for each takeoff. Remember, a jet engine is nothing more than a big air pump, and anything that affects the density — and consequently the weight — of the air entering the engine inlet will affect the power available. The power calculation results include maximum power for takeoff, normal climb power, and maximum continuous power. When power is set for takeoff, the first limit reached will be the limiting parameter for takeoff. In turboprop aircraft it's commonly termed temps, turns, or torque — meaning the first parameter reached among engine temperature, engine rpm, or engine torque (torque is pertinent only to turboprops) will be the limiting parameter for takeoff. Normally in a jet engine, the N 1 rpm (or EPR in a pressure-rated engine) will be reached before temperature limits are approached, but in an engine nearing TBO (time between overhauls) or having sustained some damage or wear, temperature limits may be reached first. This is an excellent indicator that maintenance attention is required.

Pushed to the max

New jet pilots notice that maximum power allowable is often used for cruise operations unless maximum endurance is needed. Because of the physics involved with air compression and movement, the power available from a jet engine does not increase in a one-to-one ratio with increasing engine speed. In fact, it's more of an exponential curve, and the top 30 percent of power available from the engine is produced in the top 10 percent of engine rotation. If you've ever wondered why jet pilots hate noise-abatement procedures that have them making large power reductions on the initial climbout, this is the reason.

Linda Pendleton, AOPA 525616, has accumulated more than 10,000 hours in her 27 years of flying and has given more than 4,000 hours of jet instruction.