Question from two slots above: "... the sister [-ship] ... break up, May 76, ... provide some more details ..."
The US Safety Board provided an NTSB-AAR-78-12, on web at
http://www.airdisaster.com/accrep/#1978
Iranian Air Force flt
ULF48, 9May76 daylight 1430 GMT,
B747-131 (cargo conversion) to McGuire from Tehran, inflight structural failure, inflight breakup, crashed near Madrid Spain.
Left wing separated at an altitude of 6000', in severe wx and TRW, and Electrical Storms. FDR inop at time of accident. You can read the CVR analysis on AAR pg 5+.
A/C purchased from TWA on 1Mar76
[along with a second B747 later returned to TWA (really never left Boeing-Wichita) suffering the later 17Jul96 accident], large cargo door installed at Boeing-Wichita. Left Wing showed no evidence of pre-failure damage, nor stress corrosion, nor fatigue. Left Wing had three separate span-wise failure locations.
Board examined three possible causes:
* possible internal over-pressure in fuel tank;
* possible turbulence loads; or
* possible dynamic loads from whorl of engine pylon.
Damage indicated explosion, possible P.C.:
-- first, ignition of fuel vapors in #1 fuel tank w/over-pressure.
-- 2nd, integrity of aft wing lost, reduced torsional strength; then oscillation (flutter) of wing and engine pylon, then compression fracture of upper structure of left wing.
Witnesses rept'd lightening strikes (but vent outlet Surge Tank Protection had NOT activated, suggesting that lightening didn't hit near vent). [Note, TWA had installed STP on various models.]
Report listed five lightening accidents (Constellation near Milan Italy, B707 near Elkton MD, USAF KC-135 near Madrid, KSC FL USAF F-4, Pacallpa Peru L-188) all attributed to Lightening Strike to a wing followed by explosion in same wing.
In this B747 case ignition at vent outlet was not found, prompting investigators to list other strike routing-ignition paths.
No final P.C., only hypotheses of Lightning or Turbulence.
Investigated by US-NTSB, NTSB-AAR-78-12.
Recall that only a few months later, the C141 inflight breakup over London -- records show investigators comparing notes re' Turb vs Lightning, in each case.