DENVER -- It's one of several models of World War-Two vintage aircraft used to dump slurry on forest fires.

The question: Was Thursday's crash an accident? Or an accident waiting to happen?

7NEWS Investigator Tony Kovaleski located an internal Forest Service memo raising many questions

Kovaleski: This memo is not brand new. In fact it was written back in June 1995. It raises serious questions about some of the aircraft and some of the private contractors paid by the Forest Service to provide, operate and maintain these plans.

Kovaleski: A Forest Service aviation maintenance manager authored the memo. In it, he raised questions about the quality and regularity of maintenance.

"In many cases the only time a mechanic is sent is when it is so bad the crew cannot fix it," the memo states.

His report followed inspections at five private aircraft firefighting Facilities including Hawkins and Powers in Wyoming, the company that owned the plane that crashed yesterday. And also the one that crashed last month while fighting a California forest fire.

The wife of the pilot killed in this crash also questioned the maintenance and the equipment.

"It's absolutely ridiculous to put the pressure on these men and women to fly these planes and provide them with shoddy equipment and shoddy maintenance and expect people to perform as national assets with pieces of junk," said Laurie Labare.

Kovaleski: Here's another key point from that 1995 Forest Service memo: the aviation manager said "we need to take a close look at the airtanker program and attempt to get some of the loose ends tightened up."

Kovaleski: We do not know exactly what happened yesterday, but it is clear that many have raised questions about the maintenance and airworthiness of many of these planes.

Kovaleski: And it is clear the Forest Service has been aware of this problem for several years.