FedEx Express experienced a weaker second quarter than anticipated as a 6% increase in costs outpaced a 4% increase in revenue growth.

The airfreight unit’s operating costs totaled $6.6 billion for the quarter, while revenues reached $6.9 billion, the company reports in its quarterly earnings statement. The biggest jump among operating costs came from purchased transportation, which grew 36% from the same quarter last year.

Operating income for the three-month period fell 33% year-on-year to $230 million.

Executives for the Memphis-based shipper blamed several factors for the downturn. “At FedEx Express, persistent weakness in the global economy and increasing demand for lower-yielding international services limited profits during the quarter,” FedEx Corp. CEO Fred Smith says during an earnings call.

CFO Alan Graf, meanwhile, points to the negative impact of year-over-year net fuel changes, higher costs association with depreciation and pensions, and losses from Hurricane Sandy.

FedEx Express David Bronczek during the call noted, “You had those significant headwinds in our profits and our margins this quarter,” although he added, “Going forward we’re very optimistic that those margins and profits improve.”

FedEx Corp. is maintaining its commitment to a $1.7 billion boost in profitability by fiscal 2015, primarily through $1.6 billion in measures at FedEx Express. The company will proceed with a voluntary buyout program at the airfreight unit now expected to cost $550-$650 million. The costs of the buyouts will likely reflect in fourth quarter earnings, FedEx says.

Other changes are in store for Express, which has placed an order for four additional Boeing 767 freighters as part of its fleet modernization plan. The 767s will replace MD-10s. Graf during the conference call also said Express will defer deliveries of two Boeing 777Fs from fiscal 2015 to fiscal 2016.

A FedEx spokesman says all four 767s will be delivered in fiscal year 2015.