Relatives of the five crew members aboard a Boeing 737 cargo plane that crashed into the Arabian Sea off Karachi last week are urging an international search effort to find the flight recorders to determine the cause.

Debris from the K2 Airways freighter was recovered shortly after the July 7 crash, but the water in the area is about 3,000 metres deep.

Finding the “black boxes” would require a costly underwater search likely to need foreign assistance, according to aviation experts familiar with deepwater crashes such as Air France 447 in 2009.

The locator beacons on the 27-year-old plane were designed to transmit pings for only 30 days.

Recovering the recorders could show whether a navigation system issue reported shortly before the crash was linked to a navigation component that relatives say was replaced before the flight.

The Pakis­tan Airports Authority has not provided an update on the search operation for a week, and an industrial company with underwater search expertise told Reuters it had not heard of any requests by Pakistan for assistance from foreign companies or navies.

“The search has to continue, and whatever resources can be deployed, locally and internationally, should be deployed,” Yashib Rizwan, the eldest son of Captain Rizwan Idris, told Reuters.

“For us, a transparent investigation is key,” he said.

Engineer Muhammad Arif Siddiqui’s son, Abdur Rafay Siddiqui, also called for international assistance if needed.

Both families have held funeral prayers after losing hope that the bodies would be recovered.

The government has not responded to questions about whether it will seek foreign assistance to search for the plane.

K2, which lost its only plane in the crash, has not responded to requests for comment.

Navigational system issue

The pilots reported a navigational system issue at 9:18 pm while flying to Karachi from Sharjah in the United Arab Emirates, the PAA said last week.

Local air traffic control tried to guide it, but three minutes later, radar systems showed the plane descending rapidly and communication was lost, it said.

Flightradar24 data showed the plane plunged about 5,000 feet in less than a minute, climbed about 6,000 feet in 30 seconds and then entered a catastrophic dive from 36,550 feet.

The plane spent about 10 days in Sharjah before the flight while the pilots awaited a replacement part from the US after a maintenance fault, said Ghulam Nabi, the father-in-law of co-pilot Faisal Jatoi.

One of the plane’s two inertial reference units (IRUs), which feed information on the aircraft’s position, speed and orientation to the cockpit displays, was replaced in Sharjah, the captain’s son Yashib Rizwan said.

“If you have a problem with your IRU, you just can’t rely upon the instruments,” said John Goglia, a former US National Transportation Safety Board member, adding that pilots flying at night over the ocean without visual references could struggle to determine the aircraft’s orientation.

Aircraft accidents are usually caused by multiple factors, and it remains unclear whether the replacement of the IRU is related to the crash.

An inertial reference system malfunction contributed to the 2007 Adam Air crash in Indonesia, where investigators found the pilots became fixated on troubleshooting erroneous information, failed to notice a steep right bank and lost control before the aircraft plunged into the sea, killing all 102 people aboard.

Pings from the Adam Air black boxes were detected about three weeks after the crash in a search aided by the US Navy, but recovering the recorders from about 2,000m of water took a months-long multimillion-dollar effort using a specialised remotely operated vehicle.

US aviation expert Todd Curtis said on the “Flight Safety Detectives” podcast that Pakistan was unlikely to mount a similar recovery operation unless there was a compelling reason given the K2 plane was an ageing cargo jet rather than a current-production passenger model.