Purpose and Operation
The takeoff warning inhibit prevents flight crew distractions during takeoff. The takeoff warning inhibit prevents some warning indications and some alert indications.
All channels in each WEU calculate the takeoff warning inhibit.
The takeoff warning inhibit starts when computed airspeed (VCAS) goes to the decision speed (V1), or pitch angle is 5 degrees or more.
The takeoff warning inhibit stops when the radio altitude goes above 400 feet, or 25 seconds after either the pitch angle was 5 degrees or the airplane reaches V1.
Warnings and Alerts Prevented
The takeoff warning inhibit prevents these warnings and alerts:
*
Cabin pressure
*
Fire warning
*
Altitude alert
*
Overspeed
*
Takeoff configuration warning.
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WES -TAKEOFF WARNING INHIBIT
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WES -WARNING/ALERT VOTING LOGIC
General
Each warning electronic unit (WEU) channel has 2 of 4 voting logic.
Before a WEU gives a warning or an alert output, the 2 of 4 voting logic makes sure that at least two channels give the same output. This prevents warning electronics system errors.
For example: If channel left 1 (L1) detects a fire warning condition, it will not make a warning alert output (master warning lights on and firebell aural on), until at least one other channel detects the same condition.
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WES -ALERT AURAL GENERATION
Alert Aurals
The aural synthesizer control function of the WEU gives and amplifies alert aurals. These are the alert aurals, in order of priority:
*
Siren
*
Fire bell
*
Caution
*
Chime.
Siren
For all EICAS warning messages, except fire, the WEUs give the siren aural. The siren is a sound that alternates between 250 Hz and 950 Hz. The siren has the highest priority.
Fire Bell
The WEUs give the fire bell aural when there is a fire. The fire bell is a bell sound that is on for 0.85 seconds and off for 9.5 seconds. The fire bell is second in priority.
Caution
The WEUs give the caution aural when there is an EICAS caution message. The caution is four tones one after the other. Each tone is 160 msec long. Each tone has a constant amplitude and a frequency that varies from 20 Hz to 950 Hz. The caution aural is third in priority.
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Chime
The WEUs give a high/low chime when there is a call to the flight deck, or when a comm low or medium message shows. The high/low chime is two tones one after the other. The first tone is 1160 Hz with an amplitude that goes down with time. The second tone is 580 Hz with an amplitude that goes down with time. The chime aural is the lowest in priority.
GPWS, PWS and TCAS
The WEUs do not make the GPWS, PWS, and TCAS alert aurals. The GPWS, PWS, and TCAS supply analog alert aurals to the WEUs. The WEUs amplify these alert aurals. The GPWS, PWS, and TCAS alert aurals have no priority. They go on top of other WEU alert aurals.
Alert Aural Logic
Each channel in each WEU makes the same alert aurals.
The alert aural logic makes alert aural requests. The requests are part of the warning and alert conditions for the WEUs.
When the alert aural logic makes an alert aural request, it also makes an aural arm signal. This signal goes to the other three WEU channels. The aural arm signal also permits the WEU channel to make alert aurals.
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WES -ALERT AURAL GENERATION
Alert Aural Priority Logic
The alert aural priority logic receives alert aural requests from the alert aural logic.
If there is more than one alert aural request at a time, the alert aural priority logic decides which alert aural has the higher priority.
Only the higher priority alert aural request goes through the alert aural priority logic.
Two of Four Vote
The alert aural requests go to the aural synthesizer only when at least one other WEU channel supplies an aural arm signal.
Aural Synthesizer
The aural synthesizer makes the alert aurals. Gain control circuits set the aural synthesizer audio gain.
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