Originally Posted by
WHBM
My understanding is that it's not the airframe itself that is the principal reason for retirement (and it not having any operational secondhand value), but all the fittings, the wiring especially, the control runs, the need for cabin refreshes, the IFE becoming outmoded, etc. This becomes cumulative over many of these items as time passes, and progressively impacts on dispatch reliability. It particularly applies where some of the hundreds of initial suppliers of these smaller components have gone out of business over the years, and spares and support for them becomes increasingly expensive or difficult.
Short answer, it's everything. Back in the 707 days, aircraft were designed for about 20 years, 60,000 hrs (if you do the math that's about 8 hours/day average utilization). Some of the European authorities certified to something called 'Safe Life' - basically when the airframe reached that number of hours or cycles, it was effectively grounded. Eventually though, some operators wanted to keep flying high time aircraft past those limits, despite some crashes due to fatigue related structural failures, The FAA and other authorities, with the cooperation of the airframers, looked at what it would take to keep an aircraft flying safely past it's 'design lifetime'. Eventually the idea of 'design lifetime' and 'safe life' were discarded - the position now days is that you can keep flying an airframe indefinitely - IF it's properly inspected and maintained - and enhanced inspection and maintenance procedures were developed and published to support the operation of very high hour/cycle aircraft.
Obviously these maintenance and inspection requirements go far beyond primary structure. While this part didn't get much press, TWA 800 was (at the time) one of the highest time aircraft ever - over 100,000 hours - which no doubt contributed to the wiring issues that are believed to have causing the fuel tank explosion.
At some point, the additional costs related to the extra maintenance and higher fuel burn make it uneconomical to keep flying an old aircraft instead of investing in something newer. OTOH, lots of seriously old DC-3s, 727s, 737-100/200, DC-10s, and 747-100/200/SP are still plying the skies - either in specialized roles (I was just in Las Vegas, there is a 747SP parked there that is apparently used by one of the big casinos to ferry in some of the high roller types), or operating in areas where the labor costs are low and regulatory oversight is rather lax...