IF SOME ICE MAKE THE RUDDER TO LOSE ITS FUNCTION - VOEPASS CRASH ATR-72-500 IN BRAZIL

 ‘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

SOURCE:

AIRCRAFT DESIGN

A Systems Engineering Approach

Mohammad H. Sadraey

Daniel Webster College, New Hampshire, USA

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

flights, it will be considered to be at level 2. If any aircraft has one failure per 100 flights, it is considered to be at level 3. An aircraft in level 3 is recommended to be retired to avoid an accident, because any time a system or component fails, an accident may occur. The control surfaces must be designed such that the level 1 of handling qualities is achieved.

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;

and helix radius (R), half of the wing span.

 The design of the rudder

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

left (of the pilot). A positive rudder deflection creates a positive side force (i.e., in the positive y direction) but results in a negative yawing moment (i.e., counterclockwise).

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

E – Elevators forward to break the stall

For a flight instructor it has no relevance icing condition on the aircraft. The ICE & RAIN PROTECTION SYSTEM was developed to keep the plane from icing condition. 

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.  

 

Real spiral flat spin training overview

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.

On the contrary, the normal spiral spin descent flight, the aircraft nose will fall pointing directly to terrain. The plane makes all descent turns very near to the vertical spiral axis. It starts the first turn with 10 meters of radius and at the final turn the airplane will make a turn with 10 meters of radius centered on spiral vertical axis.

Spin Recovery

One of the most important roles of a rudder in 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 vertical (z) axis, during which the mean angle of attack of the wings is beyond the stall. Almost since man first flew, spinning has caused many fatal accidents, so that most accidents were due to spin. During years 1965 to 1972, US Navy has lost an average of 2 aircraft per month and total of 169 aircraft due to spin, the list of which is headed by 44 fighter aircraft F-4s (Phantom). This statistics show the crucial role of the rudder in a spin.

Spin is a high angle of attack/low airspeed situation; the airspeed will be hovering somewhere down in the stall area. Spin has two particular specifications: 1. Fast rotation around vertical axis, 2. Fully stalled wing. Spin is usually starts after wing stalls. One of the reasons why aircraft enter into spin is that inboard of the wing stalls before outboard of the wing, in other word, lift distribution over the wing is not elliptic. Spin is recovered by a procedure which all control surfaces (elevator, aileron, and rudder) contribute; particularly the rudder in an apparently unnatural way. The rudder is the most significant element is spin recovery to stop rotation. The primary control for spin recovery in many airplanes is a powerful rudder.

The rudder must be powerful enough to oppose the spin rotation in the first place. A spin follows departures in roll, yaw and pitch from the condition of trim between the predominantly pro-spin moment due to the wings and the generally anti-spin moments due to other parts of the aircraft. If spin is not recovered, aircraft will eventually crash. The criterion for rudder design in a spinnable aircraft may be spin recovery. Acrobatic and fighter airplanes are usually spinnable, but there are some airplanes such as some transport aircraft that are spin-proof or un-spinnable.

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.

 Basically, the rudder is not the only factor to feature an acceptable spin recovery. Two other significant factors are as follows:

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.