LESSON 3

WARNING AND RECORDING SYSTEMS

 CONTENT

1. ALTITUDE ALERT SYSTEM

2. OVERSPEED WARNING
3. STALL WARNING
4. FLIGHT DATA RECORDER
5. REVIEW
1. ALTITUDE ALERT SYSTEM
Air traffic control separates the aircraft during flight by
clearing them to different flight altitudes. Therefore it is
very important that an aircraft flies only at an altitude
which is cleared by air traffic control, to avoid collisions.
A deviation from this clearance altitude is detected by
the Altitude Alert system which alerts the pilot with a
visual and aural alert. The altitude alert system compares
the actual altitude delivered by the air data computer
with the clearance altitude from ATC, which the pilot
must always select on the altitude window
A typical altitude alert system alerts the pilots when the
aircraft deviates more then 300 fts from the selected
altitude. This is called the deviation mode. The alert
consist of a short chime from a loudspeaker in the
cockpit accompanied by a flashing altitude alert light.
The visual altitude alert extinguishes if the aircraft
returns to the correct altitude.
Before an aircraft is allowed to climb or descend the
pilot must ask ATC for a new clearance altitude which
then must be selected with the altitude knob. During this
selection the altitude alert is inhibited, so the deviation
alert will never appear when the pilot follows the
correct procedure.
A new altitude selection is not done when during an
approach the aircraft intercepts the glideslope to start
the landing. Therefore in this situation the altitude alert
is inhibited by the system so that the pilot is not
confused. Typical altitude alert systems also give an
alert when the aircraft approaches the selected altitude
during manual flight. This is to remind the pilot to level
off at the correct altitude.
2. OVERSPEED WARNING
The airspeed indicator shows the actual indicated airspeed
and a speed limit. In jet aircraft this is done by maximum
operating velocity (VMO) in the airspeed indicator or
with the red and black area on the speed tape of the PFD.
All jet aircraft have an independent aural overspeed
warning, because of the danger of aircraft damage during
an overspeed. It is an aural warning from the cockpit
loudspeakers which is triggered by the master warning
system. The overspeed warning is generated whenever
the airspeed is higher than VMO. The detection is always
independent of the airspeed indication.
III. STALL WARNING
Aircraft can only fly if the wing generates sufficient lift,
which depends mainly on the wing area , the airspeed
and the angle of attack. To keep a constant lift as the
airspeed decreases you must increase the angle of attack
or you must change the wing area by extending the slats
and flaps.
At a certain angle of attack, called the alpha max, the
airflow cannot follow the upper surface of the wing
and an airflow separation occurs. This stall condition
is very dangerous because the lift decreases
dramatically and the aircraft crashes if not enough
altitude is available for recovery. Therefore the pilot
must be warned early enough before the real stall
happens. This is the task of the stall warning system.
The simplest form of AOA indicator is a stall warning
device that does not have a gauge located in the cockpit.
It uses an aural tone to warn of an impending stall due to
an increase in AOA. This is done by placing a reed in a
cavity just aft of the leading edge of the wing. The
cavity has an open passage to a precise point on the
leading edge.
In flight, air flows over and under a wing. The point on
the wing leading edge where the oncoming air diverges
(separate) is known as the point of stagnation. As the
AOA of the wing increases, the point of stagnation
moves down below the open passage that leads inside
the wing to the reed.
 Air flowing over the curved leading edge speeds up
and causes a low pressure. This causes air to be sucked
out of the inside of the wing through the passage. The
reed vibrates as the air rushes by making a sound
audible in the cockpit. Another common device makes
use of an audible tone as the AOA increases to near the
point where the aircraft will stall.
This stall warning device includes an electric switch
that opens and closes a circuit to a warning horn audible
in the cockpit. It may also be wired into a warning light
circuit. The switch is located near the point of
stagnation on the wing leading edge. A small lightly
sprung tab activates the switch.
At normal AOA, the tab is held down by air that diverges
at the point of stagnation and flows under the wing. This
holds the switch open so the horn does not sound nor the
warning light illuminate. As the AOA increases, the point
of stagnation moves down. The divergent air that flows
up and over the wing now pushes the tab upward to close
the switch and complete the circuit to the horn or light.
A true AOA indicating system detects the local AOA of
the aircraft and displays the information on a cockpit
indicator. It also may be designed to furnish reference
information to other systems on high-performance
aircraft.
The sensing mechanism and transmitter are usually
located on the forward side of the fuselage. It typically
contains a heating element to ensure ice-free operation.
Signals are sent from the sensor to the cockpit or
computer(s) as required. An AOA indicator may be
calibrated in actual angle degrees, arbitrary units,
percentage of lift used, symbols, or even fast/ slow.
There are two main types of AOA sensors in common
use. Both detect the angular difference between the
relative wind and the fuselage, which is used as a
reference plane. One uses a vane, known as an alpha
vane, externally mounted to the outside of the fuselage.
It is free to rotate in the wind. As the AOA changes, air
flowing over the vane changes its angle.
The other uses two slots in a probe that extends out of
the side of the fuselage into the airflow. The slots lead to
different sides of movable paddles (cánh) in a chamber
of the unit just inside the fuselage skin.
As the AOA varies, the air pressure ported by each of the
slots changes and the paddles rotate to neutralize the
pressures. The shaft upon which the paddles rotate
connects to a potentiometer wiper contact that is part of
the unit.
The same is true of the shaft of the alpha vane. The
changing resistance of the potentiometer is used in a
balanced bridge circuit to signal a motor in the indicator
to move the pointer proportional to the AOA.
Modern aircraft AOA sensor units send output signals to
the ADC. There, the AOA data is used to create an AOA
indication, usually on the primary flight display. A stall
warning system compares the actual angle of attack
from the AOA sensor with the aircraft specific alpha
max. When the critical angle of attack is reached the
system activates a stick shaker motor on the control
column. It generates vibrations. In addition modern
aircraft have an aural warning
AOA information can also be integrated with flap and
slat position information to better determine the point of
stall. Additionally, AOA sensors of the type described are
subject to position error since airflow around the alpha
vane and slotted probe changes somewhat with airspeed
and aircraft attitude. The errors are small but can be
corrected in the ADC.
Stall warning system
To incorporate a warning of an impending stall, many
AOA systems signal a stick shaker motor that literally
shakes the control column to warn the pilot as the
aircraft approaches a stall condition. Electrical switches
are actuated in the AOA indicator at various preset AOA
to activate the motor that drives an unbalanced weighted
ring, causing the column to shake.
A stall warning system
In some aircrafts you can also find a stick pusher. It
automatically pushes the control column forward to
reduce the angle of attack when a stall condition is
detected. Regardless of the many existing variations for
warning of an impending stall, the AOA system triggers
all stall warnings in high performance aircraft.
4. FLIGHT DATA RECORDER
A Flight data recorder or FDR in short is required by
aviation law on all large commercial aircraft. It records
important flight data to evaluate causes of an accident.
The recorder is painted bright yellow or orange so it can
be easily located at the crash site.
The first generations of FDRs just recorded 6
parameters: These are time, heading, altitude, airspeed,
vertical acceleration and a pulse when the push to talk
switch is activated.
At now, Aircraft require a recorder which can record
a lot more data, like engine, flight control and system
status. You can see examples on the list, but modern
systems can store several hundred parameters.
Modern Flight recorders use two types of storage devices to
store the required parameters.
- One uses a magnetic tape, which is protected against heat and
shocks by insulation and
- The second type uses solid state memories.
Both recorder types do not need any maintenance activity ,
because they only store the data of the last 25 flight hours.
The flight recorder can usually be found in the tail section of
the aircraft, Typically this location is less seriously damaged in
a crash.
To keep the stored data during a crash the storage device must
be protected against high G - loads of up to 1000 gs and high
temperatures of more than 1000°C.
All Flight data recorders have an underwater Locator
Beacon, to locate the aircraft under water if it crashes
into the sea. The underwater Locator Beacon transmits
an audio signal at 40 khz that can be picked up by an
underwater microphone. Battery can use for 30 days and
FDR can withstand depths of more than 3000 m.
The flight recorder starts recording automatically at the
beginning of the flight and stops at the end. Typical
switching signals come from the engine oil pressure
switch or an airspeed signal.
A test switch in the cockpit allows a test of flight
recorder operation on the ground by bypassing the start
conditions. Modern systems can be tested via the CMC
Modern flight recorders store the data in a digital format.
All signals from the aircraft systems go first to a flight
data acquisition unit. This unit combines all data to a
recordable format and also monitors the recording.
In addition the FDR stores the flight number and the
date. It is provided either by the flight management
computer or a Flight Data Entry Panel. Here the flight
crew must enter the data manually.
FDR Operation