United's Pensions on Increasingly Shaky Ground
Analysis of United Airlines' financial problems focuses on vulnerability of its four big pension plans; says airline could decrease its debt by $1 billion by shedding just one plan; holds such drastic step could result in other airlines trimming their plans as well, to keep their labor costs competitive; says short-term prospect could be series of failed pension plans and lost benefits reminiscent of those in steel industry, costly outcome for government; says pilots' pension plan has biggest shortfall; adds airline might try to trim other worker's benefits to conserve cash; graph (M)