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  #1  
Old 09-26-2017, 01:27 PM
scsmith scsmith is offline
 
Join Date: Jan 2008
Location: Ashland, OR
Posts: 2,971
Default "interesting" wake encounter

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.
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RV-8 N825RV
IO-360 A1A
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  #2  
Old 09-26-2017, 02:20 PM
David Paule David Paule is offline
 
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Location: Boulder, CO
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Default

How fast were you going, Steve?

Dave
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  #3  
Old 09-26-2017, 02:26 PM
ArlingtonRV ArlingtonRV is offline
 
Join Date: Feb 2005
Location: Marysville, WA
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Default

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.
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  #4  
Old 09-26-2017, 03:21 PM
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turbo turbo is offline
 
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Default

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.
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  #5  
Old 09-26-2017, 03:30 PM
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az_gila az_gila is offline
 
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Question

Could a large transient negative G simply have caused the fuel pump to stop pumping for a fraction of a second?
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  #6  
Old 09-26-2017, 07:02 PM
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DanH DanH is online now
 
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Quote:
Originally Posted by az_gila View Post
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.
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  #7  
Old 09-26-2017, 08:16 PM
scsmith scsmith is offline
 
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Location: Ashland, OR
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Quote:
Originally Posted by az_gila View Post
Could a large transient negative G simply have caused the fuel pump to stop pumping for a fraction of a second?
Perhaps. Thats kind of why I asked.
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Aeronautical Engineer
RV-8 N825RV
IO-360 A1A
WW 200RV
"The Magic Carpet" Flying since Sept. 2009
Hobbs 700
also
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  #8  
Old 09-26-2017, 08:17 PM
scsmith scsmith is offline
 
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Location: Ashland, OR
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Quote:
Originally Posted by David Paule View Post
How fast were you going, Steve?

Dave
I was going normal cruise speed, at 10.5K. So probably TAS around 200 mph
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Steve Smith
Aeronautical Engineer
RV-8 N825RV
IO-360 A1A
WW 200RV
"The Magic Carpet" Flying since Sept. 2009
Hobbs 700
also
1/4 share in 1959 C-182B (tow plane)
LS6-15/18W sailplane SOLD
bought my old LS6-A back!!
VAF donation Dec 2020
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  #9  
Old 09-26-2017, 08:21 PM
scsmith scsmith is offline
 
Join Date: Jan 2008
Location: Ashland, OR
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Quote:
Originally Posted by turbo View Post
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.
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Steve Smith
Aeronautical Engineer
RV-8 N825RV
IO-360 A1A
WW 200RV
"The Magic Carpet" Flying since Sept. 2009
Hobbs 700
also
1/4 share in 1959 C-182B (tow plane)
LS6-15/18W sailplane SOLD
bought my old LS6-A back!!
VAF donation Dec 2020
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  #10  
Old 09-26-2017, 08:25 PM
scsmith scsmith is offline
 
Join Date: Jan 2008
Location: Ashland, OR
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Quote:
Originally Posted by ArlingtonRV View Post
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.
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Steve Smith
Aeronautical Engineer
RV-8 N825RV
IO-360 A1A
WW 200RV
"The Magic Carpet" Flying since Sept. 2009
Hobbs 700
also
1/4 share in 1959 C-182B (tow plane)
LS6-15/18W sailplane SOLD
bought my old LS6-A back!!
VAF donation Dec 2020
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