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AI - used for CAT I approaches

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AII - used for CAT II approaches

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AIII - used for CAT IIIa approaches.

Non-Precision (NP) Mode
The NP mode can be used during all phases of flight from takeoff to landing. This can include low visibility takeoff operations utilizing ground roll guidance. All enroute operations and approaches utilize flight director guidance and/or raw data except for flare which is always based on HUD computer guidance.
NP mode can be selected at any time and does not require and armed annunciation. It is the default mode and is displayed whenever another mode becomes inactive.
Copyright . The Boeing Company. See title page for details.
10.22.2  June 1, 2003

Flight Instruments, Displays -Head-Up Display System
Description
737 Flight Crew Operations Manual
AFDS engaged modes, autothrottle modes and autopilot status is indicated across the top of the display similar to the flight mode annunciator display. Navigation information is displayed dependent on the selected navigation source and active AFDS mode. During LNAV operations, vertical and lateral deviations are similarly displayed based on FMC data. During ILS/VOR operations, course deviation is displayed within the HSI (unless automatic switching occurs to the localizer deviation scale symbology). Glideslope data is presented on a glideslope deviation scale adjacent to the altitude tape.
A TO/GA pitch target line represents the commanded pitch attitude at rotation. The symbol has a gap near the center indicating the runway heading. To minimize the possibility of a tail strike, the commanded pitch attitude is 10 degrees until radio altitude exceeds 10 feet. Above 10 feet, the commanded attitude reverts to match CDS flight director guidance. The TO/GA pitch target line is displayed above 60 knots.
NP Mode - Low Visibility Takeoff
The NP mode includes special symbology used for a low visibility takeoff. The display supports visual runway centerline tracking and enhances situational awareness.
Note:  Approval must be obtained from the appropriate regulatory authority prior to conducting HUD low visibility takeoff operations.
The low visibility takeoff display incorporates a ground path vector symbol, and a ground localizer line. Both nav receivers must be tuned to the same frequency and the runway length must be set between 5500 and 13600 feet. The HUD derived ground localizer line provides lateral guidance relative to the ground path vector symbol to track the localizer. The localizer deviation is presented relative to the selected course mark on the horizon.
Ground roll guidance is engaged when heading is within approximately 15 degrees of selected course and groundspeed is greater than approximately 20 knots.
NP Mode - Approach and Landing
If the NP mode is used for an approach and landing, flight director guidance and navigation raw data is displayed. Once on the ground, the ground localizer line is displayed (if a valid ILS frequency is tuned on both nav receivers) to enhance centerline tracking.
Copyright . The Boeing Company. See title page for details.
June 1, 2003   10.22.3
Flight Instruments, Displays -Head-Up Display System Description 737 Flight Crew Operations Manual

AI Approach Mode
The AI mode is intended for CAT I ILS approaches. CDS flight director guidance is used to drive the HUD flight director. The HSI symbol is displayed until the localizer course is captured and the localizer deviation is less than 0.25 degrees for seven seconds. At that time there is an automatic switch to the localizer deviation scale. AI mode can be selected at any time, except in TO/GA or on the ground.
If AI mode is selected prior to capturing the glideslope and localizer, the HUD remains in NP mode with AI mode armed.
AII Approach Mode
The AII mode is intended for CAT II ILS approaches. It is similar to AI mode except that CDS flight director guidance is used to drive the HUD flight director until the glideslope and localizer are captured. At this time, the HUD switches to HUD computer guidance.
AII mode approach criteria requirements are satisfied when:
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all inputs to the HUD computer are valid, and

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IRS L in NAV mode if the IRS transfer switch is in NORMAL or BOTH
ON L, or

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IRS R in NAV mode if the IRS transfer switch is in BOTH ON R, and

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both VHF navigation receivers tuned to the same ILS frequency, and

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ILS capture criteria satisfied.

AIII Approach Mode
The AIII mode is intended for CAT IIIa approaches. It is similar in appearance to the AII mode and can be engaged at any time, up to approximately seven seconds prior to reaching 500 feet radio altitude.
　
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