AF 447 Thread No. 5

[A33Zab]

@Dozy:

UAS other than A. FBW airliner:

Unreliable Airspeed Indications.
Unreliable airspeed indications can result from blocking or
freezing of the pitot/static system or a severely damaged or missing
radome.
When the ram air inlet to the pitot head is blocked, pressure in the probe
is released through the drain holes and the airspeed slowly drops to zero.
If the ram air inlet and the probe drain holes are both blocked, pressure
trapped within the system reacts unpredictably.
The pressure may increase through expansion, decrease through
contraction, or remain constant.
In all cases, the airspeed indications would be abnormal.
This could mean increasing indicated airspeed in climb, decreasing
indicated airspeed in descent, or unpredictable indicated airspeed in
cruise.
Unreliable airspeed may cause noticeable effects in the normal speed
stability of the airplane since the normal pitch control law uses indicated
airspeed.
If the indicated airspeed falls below 50 knots, the flight control system
changes to the secondary mode, which does not depend on airspeed.
The autothrottle system also uses indicated airspeed and should be turned off.
If the flight crew is aware of the problem, flight without the benefit of
valid airspeed information can be safely conducted and should present
little difficulty.
Early recognition of erroneous airspeed indications requires familiarity with
the interrelationship of attitude, thrust setting, and airspeed.
A delay in recognition could result in loss of airplane control.
The flight crew should be familiar with the approximate pitch attitude for
each flight maneuver.
For example, climb performance is based on maintaining a particular
airspeed or Mach number.
This results in a specific body attitude that varies little with gross weight and altitude.
Any significant change from the body attitude required to maintain a
desired airspeed should alert the flight crew to a potential problem.
When the abnormal airspeed is recognized, immediately return the airplane
to the target attitude and thrust setting for the flight regime.
If continued flight without valid airspeed indications is necessary,
consult the Flight With Unreliable Airspeed/Turbulent Air Penetration table
in the Performance Inflight section of the QRH for the correct attitude,
thrust settings, and V/S for actual airplane gross weight and altitude.
Ground speed information is available from the FMC and on the instrument
displays.
These indications can be used as a crosscheck.
Many air traffic control radars can also measure ground speed.

Secondary and Direct Mode Pitch Control

Airplane pitch control is different in the secondary and direct flight control
modes.
The control columns now command a proportional elevator deflection
instead of a maneuver command.
Secondary and direct modes do not provide automatic pitch compensation for:

• thrust changes
• gear configuration changes
• turbulence
• flap and speedbrake
configuration changes

• turns to 30° bank angle.

In secondary and direct modes, the elevator variable feel system provides
two feel force levels instead of a continuous variation with airspeed.
The force levels change with flap position.
With the flaps up, the feel forces provide maneuver force levels that
discourage overcontrol in the pitch axis at high speeds.
With flaps extended (flaps 1 or greater), the feel forces decrease to
provide force levels appropriate for approach and landing.
In the secondary and direct modes, both the primary pitch trim switches
and the alternate pitch trim levers move the stabilizer directly.
There is no trim reference speed.