What most people call a "military Pension" is in reality "Half-Pay". Technically the soldier/sailor having served 20 years, is put on inactive duty at half pay. This continues till he or she dies. His widow (or her widower) has some rights, but not important here. If no 20 year service no half-pay (You may be put on half pay if you are unable to continue your military service do to an injury, which I believe, is the case with McClain, but I also believe he has his 20 years in). The down side of Half pay is the soldier/Sailor can be called up at any time for further service, since he or she is still technically "on duty" even through it is inactive.
I suspect McClain is on this program for a Navy Captain pay at present is $8913 a Month with 24 years service is $106,956yearly. Half pay would then be $53,478. Given he has a Wife, he is entailed to a spouse allowance of $1065.6 per month or $12,787.60 per year, half pay would be $6393.60 per year. Total Income at Half Pay would be $59,871.30 per month.
Now according to the account on Wikipedia he served only 23 years, excluding his time in the Naval Academy. Given that the above number is close to the Pension you give of $58,000, I fully believe he is on half-pay.
Now I do NOT know the details of how years in Service is counted, nor the actual details, the above is just an outline based on the fact that what most people call a Military Pension has been since the time of the American Revolution Half pay (and had been Half Pay in British and Colonial Service since the time of Oliver Cromwell in the mid-1600s).
For more on Pay rates see the following:
http://www.navycs.com/08militarypaychart.htmlhttp://www.militaryconnection.com/charts.aspDisability is different. That is based on how injured you are. How it relates to Half-pay I do no know, but I suspect it is reduced by how much half-pay a retiree gets (Or the half-pay is reduced by how much disability one gets). Disability goes to anyone who was injured and suffered what the VA calls a permanent injury. This is where you here the term 20%, 40$ and 100% disabled. My father was 20% disabled, but was able to work as a letter Carrier for the Post Office for almost 40 years. His wife received NOTHING from that benefit since it was viewed as the military equivalent of Workmen's Compensation.
A third program is what the Fed's call the "Improved Pension". McClain is NOT getting it for the "Improved Pension" is for those people who have served in the Military (and their spouses) and has no other source of income and no longer can work do to a disability incurred AFTER they left the service. It pays better then Supplemental Security Income (SSI) but is basically the same thing (But the DAV, VFW and American Legion have a better lobby than the Welfare Rights Organization).
Just an outline on three types of "Pension" someone who had been in the Military can receive.