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Items to Monitor During Engine Start Whether your particular engine-start sequence is handled automatically or manually, there are some important items to monitor during the starting process itself. Once you select “engine start” and monitor engine speed as rotation begins via the starter, check for some oil pressure indication prior to initiating fuel flow. (The exact oil psi varies by manufacturer.) If no oil pressure, or low oil pressure is indicated during initial engine cranking, the start sequence must be aborted. Also, monitor for EGT rise within a reasonable amount of time after initiating fuel flow, typically 20 seconds. If ignition or “light-off” does not occur within this timeframe, something is amiss and fuel flow to the engine should be discontinued. Prior to start, be sure you are familiar with the normal engine indications before, during and after the engine start sequence. Abnormal fuel flow, temperature, engine RPM or oil pressure indications could require an immediate abort of the engine start.
Turbine Engine Characteristics in Flight Turbine engines are not particularly sensitive to power changes in the air (especially as compared with turbocharged reciprocating engines) and therefore are extremely easy to manage in flight. While maximum EGT or ITT has to be monitored under certain conditions, you can readily and abruptly change thrust or power in turbine aircraft (say, from cruise to idle) with no adverse effects. Additionally, since the fuel control unit is directly controlled via the power or thrust levers, no “leaning” or “mixture” control of any kind is required. Power management of turbine engines in flight does vary from that of piston aircraft in several important ways. First, turbine engines take time to spool up when power is applied. If left at low power, engine compressor speed is slow. The term spool up refers to the time it takes, after power application, for thrust to increase from idle to the selected values. Most of a turbine engine’s thrust is developed in the top 10 percent or so of the engine RPM (N1) range. Let’s say that a jet is coasting along at flight idle (60 percent to 65 percent N1) when the pilot calls for max thrust. It can take quite a while for the engines
THE TURBINE PILOT’S FLIGHT MANUAL
to spool up to the 90 percent N1 range, where most thrust is generated. While adding full throttle in a piston aircraft may result in maximum power within a few seconds, turboprops take a bit longer, and maximum thrust in jet aircraft may not result until five to ten seconds after power application. For these reasons, being low and unspooled in turbine aircraft on approach to landing is absolutely unacceptable, due to the time it takes for the engines to spool up for go-around. (On at least one old foreign military turbojet, spool up time from idle to full power takes twelve seconds. Modern jet engines are required to spool up in five seconds or less.) Because response to thrust changes can be so slow in pure jet aircraft, power management in these vehicles requires plenty of planning. For example, on final approach in jets all of those high-lift, high-drag devices (flaps and leading edge devices) are extended to slow the aircraft enough so that relatively high thrust must be carried all the way to landing. This aids in go-around situations because, since thrust is already high, spool up time is minimized. Cleaning up the airplane turns out to be faster than waiting for spool up, especially on older turbojet aircraft. Another challenge arises in jets (less so in turboprops) because turbine engines offer relatively little drag when thrust is reduced to flight idle. While the drag of propeller discs will quickly slow a prop aircraft with engines at idle, pure jets tend to just coast. (Glide ratios can exceed 20:1 in modern jetliners.) That means thrust management to meet descent restrictions in jets can also be challenging, due to the low-drag characteristics of such aircraft. For example, it’s difficult in many jets to maintain the 250knot speed limit below 10,000 feet and to descend at the same time more than 1,500 or so fpm. This means that jet pilots must plan ahead on descents and in some cases negotiate with ATC. (“We can hold our airspeed to 280 knots or make our 13,000 altitude crossing restriction, but not both.”)
Thrust Reversers For purposes of both safety and airport size, it’s important to stop landing aircraft in the shortest distances possible. Therefore, many turbine-powered aircraft are equipped with thrust reversers to enhance deceleration upon landing. Typically, a
