‘Not icing threw to the ground the aircraft, but the spiral descent flight in flat spin’; the VOEPASS’ aircraft ATR-72-500 performing the flight 2Z-2283 outbound Cascavel, PR airport (SBCA, ICAO code) to São Paulo Guarulhos airport (SBGU) in Brazil, on Aug 9, 2024.
Spin recovery requirement
Aerodynamic balance and mass balance:
aileron, elevator, and rudder tabs of ATR-72-600
The level
of acceptability relates to the ease of flight and flight safety. According to
airworthiness standards, an aircraft with any level of acceptability from one
to three is allowed to fly, but for the design of control surfaces, level 1
must be the objective. An aircraft with level 1 can only terminate flight phase
A safely and in other phases may be run out of control. When an aircraft is in
level 1, there is no failure during phases of flight. When an aircraft has one
failure per 1 000 000 flights, it will be considered to be at level 1. When an
aircraft has one failure per 10 000
Spin
Recovery
One of the
most important roles of a rudder in the majority of airplanes is spin
recovery.
The most
significant instrument to recover aircraft from a spin is a powerful rudder.
Spin is a self-sustaining (auto-rotational) spiral motion of an airplane
about the vertical (z ) axis, during which the mean
angle of attack of the wings is beyond the stall.
The
typical range of some spin parameters is as follows:
angle of
attack (α), 30–60 deg;
rate of
descent (ROD), 20–100 m/s; [65 ft/s – 168 ft/s] {3900 ft/min – 10,080 ft/min}
rate of
spin (Ω), 20–40 rpm;
helix
angle (γ), 3–6 deg;
The rudder
is the most significant element in spin recovery to stop rotation. The
primary control for spin recovery in many airplanes is a powerful rudder.
The
convention for the positive rudder deflection is defined as the deflection to
the
four
parameters must be determined: (i) rudder area (S R), (ii) rudder chord (CR), (iii) rudder span (bR), (iv) maximum rudder
deflection (±δRmax ), and (v) location of inboard edge of
the rudder (bRi).
FAR Part
25 Section 25.147 requires
the following:
It must be
possible, with the wings level, to yaw into the operative engine and to safely make
a reasonably sudden change in heading of up to 15 deg in the direction of the
critical inoperative engine. This must be shown at 1.3 VS for heading changes
up to 15 deg, and with (i) the critical engine inoperative and its propeller in
the minimum drag position; (ii) the power required for level flight at 1.3 VS,
but not more than maximum continuous power; (iii) the most unfavorable center
of gravity; (iv) landing gear retracted; (v) flaps in the approach position;
and (vi) maximum landing weight.
The rudder
plays different roles in different phases of flight for various aircraft. Six
major
functions of a rudder are: (i) cross-wind landing, (ii) directional control for
balancing asymmetric thrust on multi-engine aircraft, (iii) turn coordination, (iv)
spin recovery,
(v)
adverse yaw, and (vi) glide slope adjustment for a glider.
Example,
Consider the
maximum allowable rudder deflection is ±25 deg. Is this rudder able to satisfy the
spin recovery requirement at 15 000 ft altitude? Assume the aircraft
will spin at an angle of attack of 40 deg.
We need to keep in mind that at 15,000 feet the RUDDER deflection demands an increase because of air density, but that deflection at that altitude must be less than 30 degrees. After all calculations we’ll get 29.11 degrees of the rudder deflection.
There is a mnemonic rule to pilots’ remembrance – PARE:
P –
Power to idle
A –
Ailerons on neutral
R –
Rudder full opposite direction of rotation
The most interesting thing in
any abnormal flight is to save the flight from the instant the abnormality has
presented to the pilot, so the pilot must be prompted to manage the abnormal flight.
An airplane only gets into spiral flat spin descent flight if
the RUDDER trim has lost its function to keep the plane flying in straight line
(forward heading). Any plane before takeoff
must have its rudder trim set to zero deflection.
To take an airplane from a diving spiral flat spin flight, you must immediately and fully push on the pedal at the same side of the highest wing, and you must keep the ailerons on neutral.
The main difference between a normal spiral spin descent flight and a flat spiral spin descent flight is the “screw thread” shape of
the descent flight.
On the flat spiral spin descent flight,
the aircraft nose keeps aligning to the Earth horizon (minimum nose up) almost
the entire descent flight, in other words, the nose does not point directly to
terrain. The airplane makes each descent turn increasing the spiral thread diameter.
If the initial descent turn has 10
meters of radius, the last turns before colliding into the terrain will have about
20 meters of radius.
Typical
range of some spin parameters is as follows: angle of attack (α):
30 to 60 degrees; rate of descent (ROD): 20 to 100 m/sec; rate of spin (Ω): 20
to 40 rpm; helix angle (σ): 3 to 6 degrees; and helix radius (R): half of wing
span. As angle of attack increases; rate of rotation increases; and helix
radius decreases.
1.
aircraft mass distribution and aircraft moments of inertia,
2.
fuselage side area and cross section.
It
is very important that the inertia term be made anti-spin (negative for right
spin) for recovery. When the magnitudes of pitch (Iyy) and roll (Ixx) inertia are
close, the effect of inertia term is little; and hence the rudder, will be
the primary control for spin recovery. But whenever the inertia term
becomes quite significant, they have a considerable impact on the spin motion,
and thus, the size of rudder. The application of
aileron to aid recovery in generally not recommended due to its nuisance impact.
In some cases, the use of ailerons while stopping a
spin may suddenly cause a spin in the reverse direction.
