Have a hatchet (not a steel-shaft estwing, a wood shaft) and a leatherman multi-tool. Then the knife need only have a 4"-6" blade for moderate cutting work, or for use as a spear. These 3 items (hatchet, multi-tool, fixed-blade) should always be included in your emergency kit when flying in a private aircraft, no matter what the flightplan is.
That said, there are many reliable fixed-blades with a 4"-6" blade.
Here is a short list of possible knives:
1) From SOG Knives: Seal Pup, Northwest Ranger, X-42 Field knife.
2) From Ka-Bar: Short USMC, Worthog.
3) From Swamp Rat Knives: Bog Dog, Bandicoot, Howling Rat.
4) From Camillus: Becker Companion, Becker Crewman Utilty, Pilot Survival.
5) From Buck: Special, Woodsman, Pathfinder, Vanguard.
6) From Cold Steel: SRK, Outdoorsman, Master Hunter.
7) From Chris Reeve: Aviator, Mountaineer, Jereboam MK VI, Green Beret.
8) From Ranger Knives: RD6
9) From Trident Knives: TCFM, Oberland
10) Any of the Grohmann Belt Knives.
11) An appropriate Mora knife, although it may not be durable enough.
Actually, that is sort of a long list. It just goes to show that there are many possibilities. Just remember: if your plane goes down, and you survive the crash, you want equipment that you can depend on.
TheSurvivalist