FAA has issued airworthiness directive 2020-21-16 for all Boeing 747-100s, -100Bs, -100B SUDs, -200Bs, -200Cs, -200Fs, -300s, -400s, -400Ds, -400Fs, 747SRs and SPs, requiring repetitive ultrasonic inspections of the upper wing skin at certain engine strut positions for cracking; repetitive detailed and ultrasonic inspections of the strut lower spar fitting, diagonal brace strut end clevis, and diagonal brace wing attach end clevis for cracking; repetitive detailed inspections of lower link fitting at certain engine strut positions for cracking; and applicable on-condition actions, effective Jan. 14, 2021.