Generic NATOPS advice about HUDs.
NATOPS INSTRUMENT FLIGHT MANUAL 15 Nov
2006 NAVAIR 00-80T-112
Natops Instrument Flight Manual
14.2.1 HUD Limitations
HUDs not endorsed as a Primary Flight Reference (PFR) may be integrated into the normal instrument cross-check, but concerns about insidious failures and its use in maintaining attitude awareness and recovering from unusual attitudes preclude its use as a sole-source instrument reference. Improvements in information integrity and failure indications have increased confidence in the reliability of HUDs; however, the combination of symbology and mechanization enabling their use as a sole-source attitude reference has not been incorporated into all HUDs.
14.2.2 Global Orientation
Many HUDs are incapable of providing intuitive global orientation information because of the small sections of space that they represent. Also, because many HUDs provide only a partial picture of the aircraft attitude, a pilot who tries to use the HUD to confirm an unusual attitude may see only a blur of lines and numbers. In a fast-moving environment, the pilot may not be able to differentiate or recognize the difference between the solid climb lines from the identical, but dashed, dive lines in the flightpath scale. Any confusion or delay in initiating proper recovery inputs may make recovery impossible.
WARNING
Unless your HUD is endorsed as a PFR, do not use it when spatially disoriented, for recovery from an unusual attitude, or during lost wingman situations; use the heads-down display anytime an immediate attitude reference is required. Typically, heads-down displays are inherently easier to use in these situations because of the larger attitude coverage, color asymmetry between the solid ground and sky, and reduced interference from the outside visual scene (glare, optical illusions, etc.).
14.2.3 HUD Field of View
HUD symbology may also obscure objects within the HUD field of view. When nonessential HUD information is displayed or when the HUD brightness level is excessive, the probability of obscuration is dramatically increased. Proper HUD settings (including elimination of non-task-essential information and adjusting the brightness to the proper level) are imperative to prevent potential hazards to safe flight.
14.2.4 Conventional Cross-Check
Pilots should remain proficient in the conventional instrument cross-check for their specific aircraft. Regardless of the type HUD you have, it is important to fly an instrument approach or accomplish a level-off occasionally without using the HUD so you retain your proficiency in the event of a HUD malfunction. Using HUD information incorrectly or at the wrong time can actually increase pilot workload, but timely, proper use of it can help you fly more precise instruments on a routine basis."