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"interesting" wake encounter

scsmith

Well Known Member
I was cruising north along the Sierra foothills, close to Auburn, when ATC alerted me to a DC-10 Heavy descending across my path 15 miles ahead of me.

The controller initially recommended a heading change 15 degrees right, which would mean that I would have crossed the flight path of the DC-10 farther behind it, and at a point where it had been higher.

I replied that I had the heavy in sight, and the controller said, "resume own navigation, caution wake turbulence."

It looked to me like I would cross the jet's flight path at a point where it was still a few thousand feet above my alt, and about 25 miles in trail. I know that the wake does descend a lot (wake modeling was a big part of my Thesis research), but I just continued on.

Sure enough, right when I was directly in trail of the DC-10, now about 25 miles away, BOOM! I hit his wake. It was a very sharp up-down-up jolt, of which I predominantly felt the 'down'. As I was getting my eyeballs tucked back in where they belong, I noticed that I had a low fuel pressure alarm. About as quickly as I noticed it, the fuel pressure returned to normal. The engine never hiccuped.

I find this interesting. The tank was within 5 gallons of full. So it is not like a big slosh would have left the fuel pickup dry. Or would it? The heave was so fast, and over with so quickly, it is hard to imagine a large air bubble at the outboard top area of the tank could quickly travel to the bottom inboard area of the tank. In order for that air bubble to move, fuel would have to move to displace it.

I wonder if the fuel pressure loss could be due to something other than the fuel pickup momentarily being in a big air bubble. Could the rapid acceleration interfere with the mechanical pump function? I don't know.

My G-meter showed +4 and -2 g's. Thats a +/- 3 g excursion from level flight.
 
I had a similar encounter in my RV-12 with a 747 going into Paine Field. I crossed at a right angle at what I thought was safe distance. I was wrong. I hit the wake also with a very abrupt up/down jolt. My head hit the canopy, my glasses and headset went flying. By the time I realized the plane was still in one piece and I had my glasses back on I saw nothing amiss on the D-180 and the engine never stumbled.

After all that I remembered that there was another one out there somewhere, so I turned parallel to the 747's course and climbed until I was above it.
 
In hind sight, what would do different. Climb, descend, deviate.

I understand there is some laser detectors that will be able to see turbulence some day.

Thanks for telling the story. :)
 
Could a large transient negative G simply have caused the fuel pump to stop pumping for a fraction of a second?
 
Could a large transient negative G simply have caused the fuel pump to stop pumping for a fraction of a second?

Probably scared the **** out of the poor thing. Steve's heart probably quit pumping for a moment too.
 
In hind sight, what would do different. Climb, descend, deviate.

I understand there is some laser detectors that will be able to see turbulence some day.

Thanks for telling the story. :)

I think the surest way to miss it would have been to turn right (opposite to direction of flight of the heavy) to put more space and more altitude difference between us. I was reluctant to do that at the time because it turned me away from the valley and headed me deeper into the Sierra. I tend to try to keep glide range to the valley when I can.
The other alternative would be to change altitude. The wake is descending (if I get a chance, I'll try to do a quick calculation of the descent rate tonight), so if I could have quickly climbed up to match altitude, it surely would have been below me.
 
I had a similar encounter in my RV-12 with a 747 going into Paine Field. I crossed at a right angle at what I thought was safe distance. I was wrong. I hit the wake also with a very abrupt up/down jolt. My head hit the canopy, my glasses and headset went flying. By the time I realized the plane was still in one piece and I had my glasses back on I saw nothing amiss on the D-180 and the engine never stumbled.

After all that I remembered that there was another one out there somewhere, so I turned parallel to the 747's course and climbed until I was above it.

Well, the two vorticies are about 80% of the wingspan apart, so for a 747, thats 211 ft. If you were cruising at 120 kt, thats just over 200 ft/s. So you would have transited the entire wake in one second. Your turn would not have done anything to avoid the second vortex. Unless you meant there was a second 747 lurking out there.
 
I would say that if you take a partially full 2 litter bottle and give it the same type of transients you would see the liquid separate out of one big blob to lots of little ones. A fuel pickup moving through all this would have trouble finding just fuel.

My 2C

Pete
 
Steve, from your profile it appears that you have an injected IO-360. What are your setting for fuel pressure warnings?

It would seem to me that it wouldn't take much in the way of sudden onset/"offset" of G to very momentarily trigger an alert. Not suspecting unporting, but I suppose that's possible. Perhaps a short "slug" of air in the fuel line due to the brief negative G? Obviously, that would take several seconds to reach the fuel pump(s). So if the low-pressure warning was nearly instantaneous, then perhaps the sudden G-force changes affected the fuel flow just enough to trigger the warning.

The other thought is that you have waay too much instrumentation! :D

PS: Thanks for all your expert contributions to this forum......
 
Steve, from your profile it appears that you have an injected IO-360. What are your setting for fuel pressure warnings?

It would seem to me that it wouldn't take much in the way of sudden onset/"offset" of G to very momentarily trigger an alert. Not suspecting unporting, but I suppose that's possible. Perhaps a short "slug" of air in the fuel line due to the brief negative G? Obviously, that would take several seconds to reach the fuel pump(s). So if the low-pressure warning was nearly instantaneous, then perhaps the sudden G-force changes affected the fuel flow just enough to trigger the warning.

The other thought is that you have waay too much instrumentation! :D

PS: Thanks for all your expert contributions to this forum......

You are likely correct.

My normal pressure is 23-25 psi. I have a yellow warning at 18psi and a red warning at 15 psi. When I got the warning, and looked at the display, it was showing 12 psi, but rapidly climbed back up over a period of less than 5 seconds to normal.

Just an EMS and a G meter. :D
 
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