Definition
A manually operated backup pump installed in some retractable-gear airplanes that allows the pilot to generate hydraulic pressure by hand when the normal engine-driven or electric hydraulic system has failed. It is most commonly used to extend the landing gear in an emergency.
Plain English
A hand-powered pump in the cockpit that the pilot can work by hand to push hydraulic fluid through the system when the regular pump is not working. It gives the pilot a backup way to lower the landing gear.
Context Anchor
Seen in emergency gear extension procedures for airplanes with retractable landing gear.
Derivation
Auxiliary' comes from the Latin auxilium, meaning 'help' or 'aid.' In aviation it is used for any system that helps or backs up the main system. So an auxiliary hand pump is literally a 'helping hand pump' -- a backup operated by hand.
Why Pilots Care
Allows the pilot to complete a normal landing instead of a gear-up emergency landing when primary systems are inoperative.
Intuition Check
Do not assume “auxiliary” means optional or unimportant. Here it means a backup system used when the normal system is not doing the job.
Example Sentence 1
After the hydraulic pressure dropped to zero, the pilot followed the checklist and used the auxiliary hand pump to lower the landing gear.
Example Sentence 2
The checklist directed the crew to use the auxiliary hand pump if the gear failed to extend after three attempts with the normal system.