June 1, 2007
By Barry Schiff
Aviation writer Barry Schiff retired from TWA in 1998.
Considering the proliferation of very light jets being introduced, there are going to be many pilots flying jet airplanes for the first time, and some have asked me what it is like to fly a jet.
"It's easier than you might imagine," I tell them truthfully.
Think about it. Each engine has only one control, a thrust lever. Gone is the cluster of knobs to adjust propeller pitch, cowl flaps, mixture, or alternate air sources. The preflight runup is simple because there isn't one. You don't even have to warm up the engine. Just light the fire and go. This is true when operating any jet engine no matter how small or large.
But there is more to it than pushing the "go knob." Handling characteristics are different primarily because there are no propellers. Without a propeller to blow gobs of air across the wings and preserve lift, there is no difference on a jet between power-on and power-off stall speeds. When flying piston airplanes, varying power causes the nose to rise or fall. But the absence of prop wash across the tail of a jet airplane means that changing power has little effect on pitch.
Nor is there any p factor, the force that causes propeller driven airplanes (especially singles) to yaw when flown at high power and large angles of attack. You can climb steeply in a jet with feet flat on the floor, a reason that aerobatics are much easier in jets.
The high altitude capability of turbine aircraft leads to the misconception that jet engines develop rated power at altitude. This is incorrect. A turbine engine is normally aspirated and loses power with altitude. At 36,000 feet, for example, a jet engine produces only one fourth of its rated power. This is why climb rates decrease with an increase in altitude. A heavily loaded widebody often struggles at 100 to 200 fpm to reach a lofty altitude.
A nice feature about jet engines is that ignition is unneeded after engine start; the fire in the burner section is self-sustaining as long as there is an uninterrupted supply of kerosene. It is like a bonfire that continues for as long as you add logs.
The propellers of piston powered aircraft create enormous drag at idle power, so that reducing power in a piston airplane results in rapid deceleration. The windmilling propeller acts like an air brake. But retarding the thrust levers of a jet airplane does not result in such aerodynamic braking. Without windmilling propellers, deceleration is hardly noticeable. You almost have to study the airspeed indicator to detect changes. This is why turbine aircraft have spoilers. "Popping the boards" is the only way to slow up and go down at the same time.
Another advantage of turbines is that they are impervious to the shock cooling said to be harmful to piston engines. A jet pilot can close the throttle nonchalantly and without concern. This is why descents in a Boeing 747 or a Cessna Citation are often made at idle power. A descent is a prolonged glide.
Without propeller drag, turbine aircraft have outstanding glide performance. Most jets — even jumbos — can glide along a 3-degree glideslope with engines idling. This represents a 20-to-1 glide ratio, twice that of a typical propeller driven airplane. Multiengine turbine airplanes are easier to control than piston twins following an engine failure; after checking yaw, there is no rush to feather a propeller.
During the 1950s when airline pilots first learned to fly jets, there was concern about the jet engine's inability to spin up quickly from idle and provide power on demand. During a rejected landing, the pilot advanced the throttles from idle but had to wait four to six critical seconds for the engines to "spool up" and deliver power. On a few occasions the power came too late and caused some spectacular accidents.
Modern turbine engines are turbofans. These are more responsive than "pure jets" because the outer sections of the compressor blades of turbofans are ducted fans — like high-speed propellers — that "bite" the air and produce thrust almost as soon as the throttles are advanced. Turbofan engines (fanjets) also are more responsive because of their high idle speeds. Idling turbofans produce so much thrust that the pilot must ride the brakes (or use the reversers) while taxiing to prevent a lightly loaded aircraft from accelerating to a gallop. High idle also enhances glide performance. Without so much idle thrust, turbine aircraft would not glide quite as well, although they would still glide much better than propeller aircraft. Idle thrust, however, is undesirable during landing. It inhibits speed bleed during flare and partially explains why a full stall landing should be avoided. The time required to lose airspeed can consume most of the runway and not leave enough for the landing roll.
Landing a jet aircraft involves a gentle flare to arrest the sink rate and then flying it onto the runway. Spoilers are deployed after touchdown to kill wing lift and place more weight on the wheels to improve braking. The pilot then deploys thrust reversers to eliminate the effects of idle thrust.
One difficulty that a newcomer has when transitioning to a jet is learning to keep up with the aircraft. Higher speeds require thinking further ahead.
The most difficult aspect of checking out in a jet is becoming intimate with its systems and avionics. This is what truly makes flying one jet airplane different from flying another.
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