Possible Clues:

• A spell in to modify aircraft instrument
• Flyer's instrument needs change, bird being trapped inside
• It measures height above sea level
• Plane's height measurer
• Mountain climber's gadget
• Change fourth dimension within height-measuring instrument
• Change round what may be shown on dial for cockpit instrument
• Plane’s instrument’s date shown during change
• Change outside of one meteorological flight instrument
• Instrument measuring height above sea level