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SKYWEST INTERVIEW EXAM QUESTIONS AND ANSWERS
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SKYWEST INTERVIEW EXAM QUESTIONS ANDANSWERS
Negative consequences of wing sweep - ANSWER a) Lower lift necessitates the
use of high lift flaps and leading edge devices.
b) At low airspeeds, the wingtips typically stall first, resulting in a loss of aileron
efficacy. These undesirable stall characteristics make airspeed control during
takeoff and landing extremely challenging.
c) Most swept-wing aircraft have poor yaw characteristics (particularly in
turbulence) and will acquire an advanced "Dutch Roll" if not controlled with
perfect coordination. When a yaw is induced, the advancing wing offers more wing
area (span) and a higher angle of attack to the airstream, resulting in a roll in the
direction of the yaw. The lifting wing's drag causes the aeroplane to yaw and roll in
the opposite direction. In turbulence, this yaw-roll oscillation can last indefinitely.
Almost all current planes feature a "yaw damper" to counteract this tendency for
Dutch roll.
Advantages of swept wings - ANSWER a) Allows the aircraft to travel faster by
delaying the onset of the airfoil's "critical mach" by allowing the relative airflow
over its surface to move slightly outward towards the wing tips, resulting in less
apparent camber and slower acceleration over the top of the wing. This permits the
aeroplane to achieve higher speeds before reaching a critical match.
b) Jet aircraft with swept wings typically have higher cruising speeds due to a
higher critical Mach number.
What is crucial mach - ANSWER Airflow over sections of the wing (beginning at
the wing root) exceeds the speed of sound before the aircraft itself, signalling the
commencement of "mach buffet"
What is Mach buffet? ANSWER Airflow over areas of the wing, beginning at the
thicker wing root and proceeding outward toward the wing tips, begins to reach the
speed of sound, causing a shock wave that generates turbulence, disrupting laminar
airflow aft of the shockwave. Turbulence reduces the wing's lifting ability by
separating some of the airflow. If acceleration is allowed to continue, the centre of
lift advances aft as the airflow separation progresses outward and rearward, causing
,the aircraft to try to lower its nose. More trim or lift is necessary to maintain level
flight. Eventually, "Mach Tuck" will happen.
Where does stalling on a swept wing begin? - ANSWER Wingtips
Why are the wings swept on so many turbine aircraft? - ANSWER Jet aircraft with
swept wings typically have faster cruising speeds due to a higher critical Mach
number.
What is the Mach number? The ratio of TAS to the speed of sound.
What happens to sound speed if temperature or pressure decreases? - ANSWER: a)
The speed of sound changes with temperature. As the altitude rises and the
temperature falls, so does the speed of sound. Of course, the precise speed of sound
at a particular temperature is tricky due to other variables such as air composition,
humidity, heat coefficient, and so on.
b) The generally recognized value for the speed of sound at 15 degrees Celsius is
661.7 knots.
How and why do shockwaves originate before Mach 1? - ANSWER a) Air
behaves quite differently at supersonic than subsonic airspeeds. When an airplane
moves at less than the speed of sound, the air ahead of it begins to flow out of the
way before it arrives. The pressure waves formed by an airplane flying through the
air are smooth and steady.
b) However, as an airplane approaches the speed of sound and catches up to its own
pressure waves, the air ahead of it receives no warning of the aircraft's approach.
The plane plows through the air, generating a shock wave. As air rushes through
the shock wave, its pressure, density, and temperature all rise dramatically and
quickly.
What is Coffins Corner? - ANSWER The limit of an aircraft's high-altitude
operational envelope. The difference in speed between the "high-speed buffet"
(Mach Buffet) and the "low speed buffet" (stall). At extremely high altitudes, this
difference may be as little as a few knots, and for some aircraft, it may disappear
,entirely. A slight increase in bank angle or turbulence could cause a very exciting,
adrenaline-producing stall that may be irreversible.
What is a Mach Tuck? - ANSWER As the aircraft approaches Mach 1.0, the center
of lift shifts so far rearward that there is insufficient elevator control to sustain level
flight, and the aircraft may experience a deep, potentially irreversible plunge.
What is a shock wave? - ANSWER As the airplane's speed exceeds the speed of
sound, the pressure and density of the compressed air ahead of it increase, resulting
in an area of compression that extends some distance ahead of the airplane. A
shock wave is the transition from undisturbed to compressed air. This sort of wave
is created whenever a supersonic air stream is slowed to subsonic without a change
in direction, for as when the airstream is propelled to sonic speed over the
cambered region of a wing and then decelerated to subsonic speed as the area of
greatest camber is passed. A shock wave separates the supersonic and subsonic
frequencies.
Negative impacts of a shock wave - ANSWER a) drag increase: caused by airflow
separation
b) Buffet: airflow separation generates a turbulent wake behind the wing, causing
the tail surfaces to buffet.
c) Trim and stability modifications can cause Mach Tuck.
Why does the center of pressure go aft when experiencing mach tuck? ANSWER:
When the aircraft is in transonic flight and continues to accelerate, the ensuing
shock wave that forms on the wing moves aft and grows stronger.
How do winglets work? NEED TO DRAW - ANSWER a) The high pressure area
at the bottom of an airfoil pushes around the tip to the low pressure area at the top.
This movement generates a revolving flow known as a tip vortex. This vortex
travels behind the airfoil, producing a downwash that extends to the airfoil's trailing
, edge. This downwash causes a reduction in overall lift. Winglets can be attached to
the tip of an airfoil to lower flow. The winglets operate as a barrier, keeping the
vortex from developing.
b) Reduces airflow at the wing tips, which boosts lift and aircraft performance at
high altitudes.
What is a Dutch roll? - ANSWER a) A linked oscillation in roll and yaw that
becomes problematic as roll or lateral stability is reduced. During certification,
aircraft having unacceptable Dutch roll tendencies that compromise control
stability must install a stability augmentation system (yaw damper). Swept-wing jet
aircraft are extremely prone to Dutch roll, hence they are usually always equipped
with yaw dampers
b) A mixture of rolling and yawing oscillations caused by an aircraft's dihedral
effects outweighing its directional stability. Usually dynamically stable, yet
unsuitable in an airplane due to its oscillatory nature.
What feature does modern jets have to counter the Dutch roll? - ANSWER Yaw
Damper
What is a tailplane stall, and what produces them? - ANSWER a) A T-tail
arrangement can exacerbate the stall issue by providing little or no pre-stall
warning in the form of tail control surface buffet. The T-tail's position above the
wing wake remains effective long after the wing has begun to stall, allowing the
pilot to unintentionally drive the wing into a deeper stall at a considerably higher
AOA. If the horizontal tail surfaces become buried in the wing's wake, the elevator
may lose its function, rendering it unable to reduce pitch attitude and break the
stall.
b) A T-tail aircraft has a tendency to pitch up violently when stalled in extreme
nose-high attitudes, making recovery difficult or violent.
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