The storage and retirement of Boeing 737 Classics increased by nearly 25% in 2012, reflecting the aging of the fleet and difficult in finding new homes for some of the models.

An analysis using the Aviation Week fleets database shows that 114 Boeing 737-300, -400 and -500 aircraft were retired in 2012, up from 81 in 2011. That is a 41% increase.

The number of stored Classics, meanwhile, rose from 140 as of Jan. 1, 2012, to 161 by year-end, a 15% increase.

There were some series-dependent differences between the Classics.

Boeing 737-300 retirements increased from 56 in 2011 to 74 in 2012, while the number in storage climbed from 73 to 90 (including 11 at United Airlines and nine at General Electric Capital Aviation Services or GECAS). Their average age at retirement was 23.7 years.

For the 737-400, only four aircraft were retired in 2012, down slightly from the seven in 2011. But the number in storage increased from 26 to 41 (including seven at GECAS and six at BCI Aircraft Leasing) .

Retirements of 737-500 aircraft doubled to 36 in 2012, at an average age of 20.5 years. That may be partly why the number in storage dropped from 41 at the beginning of 2011 to 30 by year-end.

It is not surprising to see the number of 737-500 retirements spiking, says Jim Williams, publisher of Airfax, which lists aircraft sale and lease offerings for more than 100 marketers worldwide. The -500 is the least popular of the Classics because it is the smallest, with the highest operating cost per available seat mile, and never sold very well.

The -300 is the next largest, and the -400 the largest, so the -400 is the most desirable because it has the lowest operating costs of the three, he adds.

Also, because a lot of parts on the three versions of the Classics are common, “you would expect to see the -500s being cannibalized to maintain the -400s in operation longer.” The larger supply of spare parts lowers the cost of those parts, helping to lower the cost of operating and maintaining the -400s, he explains.

One potential outlet for the Classics is the freighter conversion market. Last October, Ireland’s Aergo Capital, a Dublin-based aircraft lessor, cited an increase in demand for Classic conversions for its decision to sign a deal with Pemco World Air Services to convert up to 20 Classic aircraft to freighters and create a new subsidiary called Aergo Cargo Solutions. Miami-based Aeronautical Engineering, an aircraft conversion specialist, also reported an increase in demand that boosted the number of Classic conversions there to about a dozen last year.