The pairing concept placed islands on opposite sides so two carriers could steam alongside one another during flight operations. Akagi and Hiryu became the only two carriers cited with port-side islands, a warning that the experiment did not survive experience. In an emergency, pilots trained around a starboard island could make a habitual control movement that turned them toward the port-side obstruction. The arrangement is linked to an unusually high aircraft-crash rate. Keeping Hiryu's boiler uptake trunks to starboard while moving the island to port also imposed structure on both sides of the hangar and flight deck, sacrificing internal and external aviation space for a formation idea.
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Hiryu's port-side island created avoidable flight hazards
Hiryu's port-side island was meant to let paired carriers operate together, but it conflicted with pilot reactions and complicated an already intrusive exhaust layout.