The Royal Navy's nominal air group counted the maximum number of fully assembled aircraft that could be stored in the hangar. It excluded boxed aircraft and did not assume that exposed aircraft could remain safely on deck in rough northern or Atlantic waters. Deck parking had cost aircraft during trials in the 1920s, although calmer conditions in the Mediterranean, Indian Ocean, and Pacific made it more practical. Illustrious also relied on Unicorn for maintenance functions that Ark Royal carried herself, another concession to weight limits. Aircraft totals therefore reflected the types carried, the operating area, and the support available.
Notable Point · 35:03-38:35
Why a carrier's aircraft total can mislead
British carrier air-group figures counted aircraft housed in the hangar, not every aircraft that calmer seas might allow on deck.