The Bhoja Air plane which crashed near Islamabad, killing all 127 people on board, was nearly three decades old, officials said on Saturday, but had been approved to fly by aviation authorities.
The Boeing 737-200 was 28 years old and had been bought on dry lease from a South African company, a civil aviation authority (CAA) official who asked not to be named told AFP.
A Bhoja Air official insisted that despite its age, the plane was safe to fly.
"The aircraft was old and second hand but it is not something unusual. The fleet of state-run Pakistan International Airlines (PIA) also runs old aircraft," Bhoja Air official Masham Zafar told AFP.
"Airlines rarely have brand new planes, and this aircraft was also refurbished.
"There was no technical issue and bad weather is to blamed. The plane left with CAA certification after normal check at Karachi airport and it was given clearance by the CAA to land at the Islamabad airport."
Bhoja Air was one of the first private airlines to set up in Pakistan after the country's skies were opened up beyond the national flag-carrier Pakistan International Airlines in 1993.
It began domestic operations in the 1990s and ran international flights as far afield as the United Arab Emirates, but had its licence suspended in 2000 after failing to pay dues to CAA.
"The company's licence has been restored in March this year permitting them to operate domestic flights after it assured us that it would repay the longstanding dues and keep up with the standard procedures," the CAA official said.
"It was the first evening flight for Islamabad, which met (with) the huge tragedy."
The official said that aside from the plane which crashed, Bhoja has another 737-200 in use and a more modern 737-400 which is awaiting CAA approval to fly.
Another CAA official said the pilot of the doomed flight Noor Ullah Afridi and first officer Javed Mushtaq joined the airline after retiring as pilots from Pakistan Air Force.