High-Altitude Aerodynamics (ATP) (AMEL, AMES)

From the pilot's viewpoint, Mach is the ratio of the aircraft's true airspeed to the local speed of sound. At sea level on a standard day (15 °C), the speed of sound is approximately 660 knots; Mach .75 there equals a TAS of roughly 498 knots. The speed of sound is directly related only to temperature — as the atmosphere cools with altitude, the speed of sound decreases, up to about 36,000 feet MSL where the standard temperature lapse stops (AC 61-107, paras 3-2a and 3-2b(2)). That single fact drives most of this Task: a colder, higher sky means any given TAS is a higher Mach number.

Two effects run in opposite directions:

• IAS falls away from TAS — an airplane's indicated airspeed decreases in relation to TAS as altitude increases, because of reduced air density; to hold the same lift coefficient, AOA must increase with altitude for the same calibrated airspeed (AC 61-107, paras 3-2b(5) and 3-3c(3))
• Mach rises against TAS — the low temperatures aloft decrease the speed of sound, so for a given TAS, Mach number is significantly higher at altitude than at sea level; compressibility effects arrive at slower indicated speeds up high (AC 61-107, para 3-3b(2))

Practically: climb at constant indicated and your Mach climbs toward the limit; that's why the climb transitions from an IAS schedule to a Mach schedule, and descent does the reverse. The result of the squeeze — slow-speed problems and Mach problems arriving at the same time — is the next card.

Coffin corner is the point where high-speed Mach buffet, IAS, and the low-speed buffet boundary merge: the airplane's absolute or aerodynamic ceiling. It forms because IAS decreases with altitude toward the low-speed buffet boundary (prestall buffet at 1.0 G) while Mach buffet arrives at ever-lower indicated speeds. There, the aircraft can neither go faster without activating the stick puller at the Mach limit, nor slower without activating the stick shaker or pusher — encountering this critical envelope region can end in loss of control (AC 61-107, para 3-2b(5)). Note the trap in the definition: the aerodynamic ceiling can sit above the AFM's certificated altitude limit, but maneuvering or turbulence effectively lowers it toward you.

AC 61-107's worked example:

• At 51,000 feet and 1.0 G: a typical turbojet might meet Mach buffet slightly above its MMO of 0.82 and low-speed buffet at 0.60 Mach — a comfortable spread
• At 1.4 G — an increase of just 0.4 G — buffet can come on at the optimum cruise speed of 0.73 Mach, and any change in airspeed, bank, or gust loading can reduce that 1.4 G protection to none
• Altitude trend: a 1.0 G buffet-free margin of 135 knots at FL350 shrinks to about 26 knots at FL450 (AC 61-107, para 3-3c(5))

Increasing either gross weight or G raises the low-speed buffet speed and lowers the Mach buffet speed simultaneously (para 3-2b(6)) — the corner closes from both sides. Hence the requirement: pick cruise altitude from the cruise maneuver/buffet limit chart, with margin for maneuvering and gusts.

Two factors, principally:

• Shock-wave-induced flow separation — normally beginning near the wing root, it decreases downwash velocity over the elevator and produces a nose-down tendency
• Aft movement of the center of pressure — the CG ends up farther ahead of the aerodynamic center than in subsonic flight, dramatically increasing the nose-down pitching tendency (AC 61-107, para 3-2b(1))

Supersonic flow over the wing is responsible for the package deal: shock waves (drag rise), the aft lift shift (Mach tuck), and airflow separation behind the shocks (Mach buffet) (para 3-3c(4)). The nose-down moment steepens the dive and accelerates you further past the limit — which is why the Mach compensating stick puller exists and must never be disabled in normal operations (para 3-2b(3); see Task I.A for the systems side).

AC 61-107's sequence for intentional or accidental excursions beyond the certificated maximum operating limit speed (para 3-2b):

• Possible onset of aerodynamic flutter
• Excessive G-loading in maneuvering
• Induced flow separation over the ailerons and elevators, potentially followed by physical loss of a control surface
• Aileron buzz or snatch, coupled with Mach tuck
• Catastrophic loss of the airplane

The convergence point of this Task: at high altitude, VMO (a structural/indicated limit) and MMO (a compressibility limit) converge with the stall AOA boundary, so both the fast edge and the slow edge of the envelope are closer than the flight deck feels. Respecting the barber pole up high is respecting a margin measured in tens of knots, not hundreds.

• Why sweep exists: sweeping the wing raises the critical Mach number — airflow travels over an effectively different cross-section with less effective camber, reducing the acceleration of flow over the wing and allowing a higher speed before critical Mach (AC 61-107, para 3-3a(3))
• Lift behavior: a straight wing builds lift steeply with AOA and then stalls abruptly with rapid lift deterioration; the swept wing produces a much more gradual buildup of lift (para 3-3a(6)) — which sounds benign but means degraded low-speed cues and deep AOA excursions
• Stall pattern: swept-wing stalls begin at the tips (AFH ch. 16) — outboard, behind the CG, aggravating pitch-up tendencies
• Fixes on the airframe: vortex generators and boundary-layer energizers delay shock-induced separation and permit a higher MMO (AC 61-107, para 3-3a(5))

A coupled oscillation in roll and yaw that becomes objectionable when roll (lateral) stability is reduced in comparison with yaw (directional) stability. When it's objectionable or affects certification control-stability requirements, a stability augmentation system is required; the yaw damper — a gyro-operated autocontrol providing rudder input — cancels the yaw tendencies (AC 61-107, para 3-3c(3)(a)). The altitude connection: reduced air density at high altitude reduces aerodynamic damping and overall stability, so jets at high altitude and high Mach can simultaneously experience slow-speed problems like Dutch roll, adverse yaw, and stall (para 3-3c(3)). Know your type's answer for a yaw damper failure at altitude — typically an altitude restriction from the AFM.

Because the ceilings that matter are margins, and both weight and temperature eat margins:

• Thrust: jets cruise at altitudes where operation is close to rpm or EGT limits — little excess thrust may be available for maneuvering, and it is often impossible to climb and turn simultaneously; all maneuvering must fit within available thrust (AC 61-107, para 3-3b(1)). A hot day moves you closer to those limits
• Buffet: higher weight raises the 1.0 G low-speed buffet boundary and narrows the corner (para 3-2b(6)), so the buffet-limited altitude falls as weight rises — and recovers as fuel burns off, which is the step-climb logic covered under Task I.B
• Efficiency: the reason to be up there at all — specific fuel consumption decreases as outside air temperature decreases, so high altitude is where fuel economy and cruise speed are best (para 3-3b(1))

L/DMAX is the minimum-drag speed: flying faster than it requires more power, and — the counterintuitive half — flying slower than it also requires more power (AFH ch. 5). Below L/DMAX the airplane sits on the backside of the power curve with speed instability: a disturbance that slows the airplane increases drag, which slows it further (AFH ch. 5). At high altitude, where excess thrust is already thin (AC 61-107, para 3-3b(1)), a slowdown below optimum speed can leave the airplane unable to hold both altitude and speed — the "high altitude slow-down" the risk elements name (AA.I.D.R3). L/DMAX is also the efficiency anchor: the smoothest, most fuel-efficient descent is idle thrust at L/DMAX (AFH ch. 16), and best-range and best-endurance speeds for your type are AFM/FMS numbers keyed off this same drag curve.

An airplane in flight unintentionally exceeding the parameters normally experienced in line operations or training:

• Pitch attitude greater than 25° nose up
• Pitch attitude greater than 10° nose down
• Bank angle greater than 45°
• Or within those parameters but at airspeeds inappropriate for the conditions (AC 120-111, para 1-6a)

That last clause is the high-altitude one — wings level, pitch normal, and 20 knots slow at FL410 is a developing upset by definition, because the airplane is diverging from the intended flightpath (AC 120-111's "developing upset condition," para 1-6e). The prevention emphasis follows: recognize the energy divergence before the attitude ever moves.

The setup is everything this Task has covered: thin thrust margins, narrow buffet spreads, reduced aerodynamic damping, and automation quietly trading speed for altitude until the airplane is behind the power curve (AC 61-107, para 3-3; AFH ch. 5 speed instability). On the recovery side, AC 120-111's warnings for the oral:

• Manage the energy state and the rate at which it is changing — it determines how much maneuvering capability you have (AC 120-111, ch. 4)
• For nose-high recoveries, pilots are instructed to push to achieve less than 1 G (AC 120-111, app. 1)
• Rudder is still effective at high AOA — and that is exactly why special care is required: guard against control reversals, and avoid rapid full-scale reversals of control deflections to maintain structural integrity (AC 120-111, app. 1). The rudder-reversal discussion continues under Task I.E
• Excessive use of pitch trim or rudder may aggravate the upset (AC 120-111, ch. 4 warnings)

Expect the scenario question as a slowdown at cruise: the correct first move is trading altitude for energy early — a descent you choose beats a stall recovery you don't.