I found a plane crash site while groping my way along the Koolau summit ridge between Manana and Waimano trail ridges one misty, cloudy, wet, typical afternoon.
As Stuart Ball says, with characteristic understatement, "The summit section there is frequently clouded over, so it is easy to become disoriented." Yeah, right! I had reached the end of Manana trail and headed towards the summit of Eleao (2654 ft, see p. 85 of Ball's book). The trail along the Koolau summit ridge is unmistakeable, has a few simple rope sections where it's narrow, and ends on top of Eleao in a nice, little, grassy meadow with a "Sound of Music" type of feel.
Continuing on towards Waimano ridge is the tricky part. If you follow the most obvious trail, you will end up on a ridge that heads right down the middle between Waimano and Manana. This is especially easy to do if it's cloudy and you can't visualize the Waihee valley below. I went about 1/4 mile along that ridge, and started to get suspicious because it wasn't as wild and windy as it had been on the top, and then the clouds lifted and I could see the summit section off to my left.
While heading back to the top of Eleao I found the plane crash site on the east side of the ridge. If you look at the topo map on Ball p. 75, the site is just to the right of the NS arrow at about 2500 ft. There's not much there, but some pieces of metal and what's left of an impact crater in the side of the ridge. It looks like the plane was headed over the Koolaus from the Kaneohe side, cleared the main Koolau ridge, and missed clearing the next ridge by about 15-20 feet.
I tried to find out more about the crash site, but the FAA and Coast Guard were all uninterested when I told them there were no dead bodies or bones, and that I couldn't tell how old the crash site was. How do you figure something like that out? But it's a very spooky experience, way up there in the wind and mist, poking around in the scraps of what used to be a plane with a pilot.
Back on the top of Eleao, the way to Waimano is down a grassy slope and along the "true" Koolau summit ridge, which should only be attempted if it's clear and sunny. It's long and hard and scary, with that long, long drop into Waihee valley inches away the whole trip.
To go up one trail and down the other, I would definitely recommend going up Waimano, because it's faster and easier than Manana, and then heading towards Manana, and back down, but ONLY IF: 1) the weather is clear, 2) it's still early (no later than noon or 1pm) when you reach the Koolau summit, and 3) the hiker knows the Manana trail end on the Koolau summit. Trying to do the hike without all three of those conditions will probably result in problems. And of course, a cellular phone is mandatory!