Airplane Flying Handbook (FAA-H-8083-3C)
Chapter 16: Transition to Jet-Powered Airplanes
Introduction
This chapter contains an overview of jet-powered airplane operations. The information contained in this chapter provides a useful
preparation for, and a supplement to, structured jet airplane qualification training. This chapter provides information on major
differences a pilot may encounter when transitioning to jet-powered airplanes. The major differences between jet-powered airplanes
and piston-powered airplanes have been addressed in several distinct areas: differences in aerodynamics, systems, and pilot operating
procedures. For airplane-specific information, a pilot should refer to the FAA-approved Airplane Flight Manual for that airplane.
Ground Safety
Stepping out on the ramp in the vicinity of jet airplanes requires special caution. There is no propeller to indicate visually whether a
jet engine is running. It is easy to inadvertently stray into danger since, even at idle, jet engines are a threat. Enough air is being
sucked into the intake to pull a nearby person into the fan. The air coming from the exhaust is hot and moving fast enough to blow a
person down.
Pilots operating jet-powered airplanes should exercise caution during taxi and when adding power to start moving. Adding too much
power can pull damaging debris up off the ground or cause damage well behind the aircraft. Jet blast when taxiing into parking areas
may affect any loose ground equipment.
Jet Engine Basics
A jet engine is a gas turbine with basic cycle of operation; that is, induction, compression, combustion, expansion, and exhaust. Air
passes through the intake and enters the compressor section, which is made up of a series of fan blades or “stages.” The first stage,
visible from the front of the engine, is the largest diameter and has the biggest blades. Each subsequent stage contains smaller
diameter and thinner blades of increasing pitch. The compression in each stage raises the air temperature and pressure. The high-
pressure hot air enters the combustion chamber where fuel is added. During engine start, igniters set the fuel air mixture on fire, after
which the fire is self-sustaining. The rapidly expanding air flows to the turbine section, which like the compressor section, consists of
a series of fan blade stages. The turbine section extracts a portion of the available energy from the airflow to turn a shaft, which drives
the compressor. The remaining energy causes rapid air expansion in the nozzle of the tail pipe, accelerates the gas to a high velocity,
and produces thrust. [Figure 16-1]
Figure 16-1. Basic turbojet engine.
The large first stage design of a turbofan engine, a ducted fan, diverts some of the air around the engine core. This cooler bypass air
produces some of the thrust. The amount of air that bypasses the core compared to the amount compressed for combustion determines
a turbofan’s bypass ratio. In a turbofan engine, the compressor and turbine sections divide into sub-sections. Each sub-section in the
turbine section connects to a specific sub-section of the compressor section via a split-spool shaft. [Figure 16-2]
Figure 16-2. Turbofan engine.
Air drawn into the engine for the gas generator is further compressed and constitutes the core airflow. While a turbojet engine uses
the entire gas generator’s output to produce thrust in the form of a high-velocity exhaust gas jet, the lower velocity and cooler bypass
air produces some of the thrust produced by a turbofan engine.
The turbofan engine design increases the thrust of the jet engine, particularly at lower speeds and altitudes. Although less efficient at
higher altitudes, the turbofan engine increases acceleration, decreases the takeoff roll, improves initial climb performance, and often
has the effect of decreasing fuel consumption.
Operating the Jet Engine
In a jet engine, the amount of fuel injected into the combustion chamber controls thrust. Because most engine control functions are
automatic, the power controls on most turbojet-powered and turbofan-powered airplanes consist of just one thrust lever for each
engine. The thrust lever links to a fuel control and/or electronic engine computer that meters fuel flow based on revolutions per
minute (rpm), internal temperatures, ambient conditions, and other factors.
Typically in jet airplanes, there are flight deck indications for the rotation speed of each major engine section. Each engine section
rotates at many thousands of rpm. For ease of interpretation, the indications read as percent of rpm rather than actual rpm. Depending
on the make and model, there are usually indications for fuel flow, as well as for gas temperatures and pressures. The associated
engine indications have different names according to their location.
As in any gas turbine engine, exceeding temperature or rpm limits, even for a few seconds, may result in serious damage to turbine
blades and other components. The pilot should monitor the temperature of turbine gases and rotation speeds as needed. Modern
aircraft are designed to prevent exceedances and alert the pilot of an impending or actual exceedance. Older designs rely more on the
pilot to prevent any exceedances.
Setting Power
When setting power, the pilot normally uses pressure or rpm indications to set maximum allowable thrust. However, the forward
movement of the thrust levers should be stopped for any limitation (e.g., pressure, rpm, or temperature).
Thrust to Thrust Lever Relationship
In a jet engine, thrust output changes much more per increment of throttle movement at high engine speeds. If the power setting is
already high, it normally takes a small amount of movement to change the power output. This is a significant difference for the pilot
transitioning to jet-powered airplanes. In a situation where significantly more thrust is needed and the jet engine is at low rpm, inching
the thrust lever forward will have little effect. It this situation, the pilot needs to make a smooth and significant thrust lever position
change to increase the power.
Variation of Thrust with RPM
Jets operate most efficiently in the 85 percent to 100 percent range. At idle rpm of approximately 55 percent to 60 percent, they
produce a relatively small amount of thrust. An increase in rpm from 90 to 100 percent may increase thrust by as much as the total
available at 70 percent. [Figure 16-3]
Figure 16-3. Variation of thrust with rpm.
Slow Acceleration of the Jet Engine
Acceleration of a piston engine from idle to full power is relatively rapid. The acceleration on different jet engines can vary
considerably, but it is usually much slower. In some cases, the transition to full power could take up to 10 seconds. [Figure 16- 4]
Pilots should anticipate the need for adding power from low power settings.
Figure 16-4. Typical jet engine acceleration times.
Jet Engine Efficiency
The efficiency of the jet engine increases in the cold temperatures found at high altitudes. The fuel consumption of jet engines
decreases as the outside air temperature decreases for constant engine rpm and true airspeed (TAS). Thus, by flying at a high altitude,
the airplane operates with improved fuel economy and speed. At high altitudes, engines may be operating close to rpm or temperature
limits, and excess thrust may not be available. Therefore, pilots should accomplish all maneuvering within the limits of available
thrust, stability, and controllability.
Absence of Propeller Effects
The absence of a propeller affects th e operation of jet-powered airplan es. Specif ic effects includ e th e absence of lif t from the
propeller slipstream and the absence of propeller drag.
Absence of Propeller Slipstream
A propeller produces thrust by accelerating a large mass of air rearward. With wing-mounted engines, this air passes over a
comparatively large percentage of the wing area. The total lift equals the sum of the lift generated by the wing area not in the wake of
the propeller (as a result of airplane speed) and the lift generated by the wing area influenced by the propeller slipstream. By
increasing or decreasing the speed of the slipstream air, it is possible to increase or decrease the total lift on the wing without
changing airspeed. Since the jet airplane has no propellers, the transitioning pilot should note the following:
1. Lift is not increased instantly by adding power.
2. The stall speed is not decreased by adding power.
The lack of ability to produce instant lift in the jet, along with the slow acceleration of jet engines, necessitates a stabilized approach
where landing configuration, constant airspeed, controlled rate of descent, and stable power settings are maintained until over the
threshold of the runway. This allows for better engine response when making minor changes in the approach speed or rate of descent
and improves go-around performance.
Absence of Propeller Drag
When the throttles are closed on a piston-powered airplane, the propellers create significant drag. Airspeed or altitude is immediately
decreased. The effect of reducing power to idle on the jet engine, however, produces no such drag effect. In fact, at an idle power
setting, the jet engine still produces forward thrust. While this can be an advantage in certain descent profiles, it is a handicap when it
is necessary to lose speed quickly. The lack of propeller drag, along with the aerodynamically clean airframe of the jet, are new to
most pilots, and slowing the airplane down is one of the initial problems encountered by pilots transitioning into jets. In level flight at
idle power, it takes about 1 mile to lose 10 knots of airspeed.
Speed Margins
Maximum speeds in jet airplanes are expressed differently and always define the maximum operating speed of the airplane, which is
comparable to the VNE of the piston airplane. These maximum speeds in a jet airplane are referred to as:
⦁ VMO —maximum operating speed expressed in terms of knots.
⦁ MMO —maximum operating speed expressed as a Mach number (the decimal ratio of true airspeed to the
speed of sound).
Mach number is the ratio of true airspeed to the speed of sound. The speed of sound varies with temperature. At low/warm altitudes,
the speed of sound is so high that an aircraft is limited by indicated airspeed. At high/cold altitudes, the speed of sound is lower so the
aircraft is limited by Mach. To observe both limits V MO and MMO, the pilot of a jet airplane needs both an airspeed indicator and a
Mach indicator. In most jet airplanes, these are combined into a single display for airspeed and Mach number, as appropriate.
It looks much like a conventional airspeed display with the addition of a "barber pole" th at automatically moves so as to indicate the
applicable speed limit at all times. [Figure 16-5]
A jet airplane can easily exceed its speed limitations. Th e handling qualities of a jet may change significantly at speeds higher than
the maximum allowed.
Figure 16-5. Jet airspeed indicator.
High-speed airplanes designed for subsonic flight are limited to some Mach number below the speed of sound. Shock waves (and the
adverse effects associated with them) can occur when the airplane speed is substantially below Mach 1.0. The Mach number at which
some portion of the airflow over the wing first equals Mach 1.0 is termed the critical Mach number (MCR).
There is no particular problem associated with the acceleration of the airflow up to the critical Mach number, the point whe re Mach
1.0 airflow begins. However, a shock wave is formed at the point where the airflow suddenly returns to subsonic flow. This shock
wave becomes more severe and moves aft on the wing as airflow velocity increases. Eventually, flow separation occurs behind the
well-developed shock wave. [Figure 16-6]
Figure 16-6. Transonic flow patterns.
If airplane speed progresses sufficiently beyond M MO, the separation of air behind the shock wave may result in severe buffeting and
possible loss of control or “upset.” Because of the accompanying changes to the center of lift, the airplane may exhibit pitch change
tendencies.
With increased speed and the aft movement of the shock wave, the wing’s center of pressure moves aft causing the start of a n ose-
down tendency or “tuck.” Mach tuck develops gradually, and the condition should not be allowed to progress to where there is no
longer enough elevator authority to prevent entry into a steep, sometimes unrecoverable, dive. An alert pilot should respond to
excessive airspeed, buffeting, or warning devices before the onset of extreme nose-down forces.
Due to the critical aspects of high-altitude/high-Mach flight, most jet airplanes capable of operating in the Mach ranges use some
form of automated Mach tuck compensation. If the system becomes inoperative, the airplane is typically limited to a reduced
maximum Mach number.
Mach Buffet
Mach buffet arises when airflow separates on the upper surface of a wing behind a shock wave. All other things being equal, shock
wave strength increases as the local airflow speed ahead of the shock wave increases. Mach buffet is a function of the speed of the
airflow over the wing—not necessarily the forward speed of the airplane, and the shock wave strength, rather than a stall, creates the
airflow separation.
Mach buffet may result from two different conditions in cruise. At high-speed cruise, a shock wave that becomes too strong as the
airflow speeds up over the upper surface causes a buffet. At low-speed cruise, the flow has a greater turn to make to follow the wing's
upper surface. The air speeds up to do that and may exceed Mach 1 over the upper surface.
The shock wave position is different between the two situations. At high speed and a lower AOA, the shock wave tends to move aft.
So when the flow separates behind the shock, that separated flow acts over a small range of the chord. In some cases, the separated
flow acting on a small surface area may produce a little buzz. At low-speed cruise, the true airspeed is still high, but the shock wave
does not move as far aft as it does in high-speed cruise. The separated flow behind the shock wave acts over a larger portion of the
chord, which leads to a more significant effect on aircraft control.
The altitude at which an airplane flying at MMO would experience buffeting with any increase in AOA determines the absolute or
aerodynamic ceiling. This is the altitude where:
⦁ If an airplane flew any faster, it would exceed MMO leading to high-speed Mach buffet.
⦁ If an airplane flew any slower, it would require an angle of attack leading to low-speed Mach buffet.
This region of th e airplane’s flight envelope is known as “coffin corner. ” Conceivably, a buff et could be the first indication of an
issue at altitude, and pilots should understand the cause of any buffet in order to respond appropriately.
An increase in load factor (G factor) will raise the low-end buffet speed. For example, a jet airplane flying at 51,000 feet altitude at
1.0 G and a speed of 0.73 Mach that experiences a 1.4 G load, may encounter low-speed buffet. Consequently, a maximum cruising
flight altitude and speed should be selected, which will allow sufficient margin for maneuvering and turbulence. The pilot should
know the manufacturer’s recommended turbulence penetration speed for the particular make and model airplane. This speed normally
gives the greatest margin between the high-speed and low-speed buffets.
Low-Speed Flight
The jet airplane wing, designed primarily for high-speed flight, has relatively poor low-speed characteristics. As opposed to the
normal piston-powered airplane,thejet winghaslessarearelativetotheairplane’sweight,a loweraspectratio(longchord/short
span), and thin airfoil shape —all of which amount to the need for speed to generate enough lift. The swept wing is additionally
penalized at low speeds because its effective lift is proportional to airflow speed that is perpendicular to the leading edge.
In a typical piston-engine airplane, V MD (minimum drag) in the clean configuration is normally at a speed of about 1.3 V S. [Figure
16-7] Flight below VMD in a piston-engine airplane is well identified and predictable. In contrast, in a jet airplane, flight in the area of
VMD (typically 1.5 – 1.6 V S) does not normally produce any noticeable changes in flying qualities other than a lack of speed
stability—a condition where a decrease in speed leads to an increase in drag, which leads to a further decrease in speed, which creates
the potential for a speed divergence. A pilot who is not aware of a developing speed divergence may find a serious sink rate
developing at a constant power setting, while pitch attitude appears to be normal. The fact that lack of speed stability may lead to a
sinking flightpath, is one of the most important aspects of jet-airplane flying.
Figure 16-7. Thrust and power required curves (jet aircraft vs. propeller-driven aircraft).
Stalls
The stalling characteristics of the swept wing jet airplane can vary considerably from those of the normal straight wing airplane. The
greatest difference noticeable to the pilot is the lift developed vs. angle of attack. An increase in angle of attack of the straight wing
produces a substantial and constantly increasing lift vector up to its maximum coefficient of lift, and soon thereafter flow separation
(stall) occurs with a rapid deterioration of lift.
By contrast, the swept wing produces a much more gradual buildup of lift with a less well-defined maximum coefficient. This less-
defined peak also means that a swept wing may not have as dramatic a loss of lift at angles of attack beyond its maximum lift
coefficient. However, these high-lift conditions are accompanied by high drag, which may result in a high rate of descent. [Figure
16-8]
Figure 16-8. Stall versus angle of attack—swept wing versus straight wing.
If a simple, straight-wing airplane’s airfoil is swept, a natural tendency arises that it will stall at the wing tips first. This is because the
boundary layer tends to flow spanwise toward the tips. [Figure 16-9] The tendency for tip stall allowing the center of lift to move
forward is greatest when wing sweep and taper are combined. To discourage a swept wing from stalling at the wingtips,
manufacturers modify the wing spanwise with twist, changes in airfoil section, inclusion of vortex generators, or a combination of
those modifications. This helps a pilot retain roll control initially if a stall is entered inadvertently.