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In Air Refueling And Turbulence

Sat Oct 19, 2002 7:49 am

so i'm watching discovery wings channel like i always do, and they have a show called "fasten your seatbelts".. today it's about the MD-11 Trijet.

during refueling that takes place at altitude, does wake turblence play a factor at all? when flying behind a heavy, we're always told to watch out for the wake turbulence and how it can toss your cessna 80degrees/second... does the same not apply to these jets? if so, why aren't they getting tossed all over the place when they're re-fueling?

RE: In Air Refueling And Turbulence

Sat Oct 19, 2002 8:19 am

I used to be a KC-135 pilot - airplanes to be refueled approach from behind and under the wake of the tanker - if they ever get into the wake, they are up to get the "ride of their life" -
The worst of the wake is generated by the wing tips of the aircraft, clockwise twist from left wing, counter-clockwise from right wing - when they are in the proper position at the end of the boom - it is smooth...
Seen some nervous guys running on fumes getting theirselves in troubles - generally I refueled B-52s... even they could get in troubles too...
(s) Skipper
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RE: In Air Refueling And Turbulence

Sun Oct 20, 2002 1:59 am

Hi Skipper,
When midair refueling, what speed do you fly at?
Is it more important to just keep the plane steady and wings level or do you try to hold a heading and altitude?
Are there conditions when you just cannot do midair refueling??

RE: In Air Refueling And Turbulence

Sun Oct 20, 2002 5:03 am

Nominal speed was 300 KIAS and FL 260 to 280... autopilot ON, level flight and level wings, steady to help the "customer"... If they wanted higher speed they could get up to 320 KIAS... we could not do much slower than 280 if we were heavy (=full fuel)...
Turbulence could be a problem for refueling, in that case you may have to descent and seek smoother air...
We accepted oil company credit cards, but did not clean the windshield  Wink/being sarcastic
(s) Skipper
T prop
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RE: In Air Refueling And Turbulence

Sun Oct 20, 2002 9:46 am

Here's a clip of a helicopter refuel gone wrong. Not sure if turbulence had anything to do with it.


T prop.
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RE: In Air Refueling And Turbulence

Mon Oct 21, 2002 2:37 am

Air to Air Refuelling (AAR) is usually done in one of 2 ways. Either the tanker flies a straight track, or flies a hippodrome pattern, the same as a Combat Air Patrol (CAP) shape. Since the performance of the 'customer' aircraft is usually higher than that of the tanker, as long as there is co-operation and co-ordination between the two, there shouldn't be any problem.

In terms of aircraft-generated turbulence, this is normally only a problem if the customer gets it wrong. There are procedures for how to approach the donor aircraft, so getting it wrong can be considered to be a blot on your copybook.

However, natural turbulence can be a pain in the rear end. I have heard from numerous pilots about trying for ages to get the probe in, repeated plugging, spilt fuel etc etc.

Just watched the Helicopter video... Oops! Bet that didn't go down well with the squadron commander!
The definition of a 'Pessimist': an Optimist with experience...
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RE: In Air Refueling And Turbulence

Tue Oct 22, 2002 6:21 am

Believe it or not, the "drafting" effect and bow wave are more of a factor than wake turbulence. Like B747Skipper said, if the closure is performed correctly, the receiver will not directly enter any wakes, but as the acft become closer, a drafting effect similar to a car behind a large truck on the highway comes into play. On smaller acft, such as fighters, the bow wave is small and the drafting only effects the receiver throttle responses, but in larger acft it is much more pronounced. As a large acft approaches, they must first break through the layer of air being created by the bow wave of their acft and the fuselage wake of the tanker. As this occurs, the closure rate is approx. 1ft/sec, the autopilot will trim out the nose down effect on the tanker, and the tanker pilot actually slowly retards the throttle to counteract the pushing effect of the receiver. The receiver acft then becomes extremely pitch sensitive and must react very carefully to prevent over-controlling the acft. When the two acft are in position, the overall aerodynamics are as if they are one aircraft, and thus the motion of one directly affects the other, causing an increase in overall airspeed and a tendency to increase closure speed due to the Bernoulli affect between the two acft.
Yes there are times when Air Refueling cannot be accomplished due to turbulence, but it is due to natural turbulence, and not acft induced.
The other factor is the weight change between the two acft. If the tanker is heavy, and the receiver is light, at the end of an AR, it is reversed, the tanker is light, and the receiver is heavy. Due to this, the performance capabilities change, and constant small corrections must be made to facilitate for this. The receiver always flies AP/AT off, and the tanker normally flies AP on and AT off, with the PF keeping one hand on the throttle and one on the yoke at all times. Practice is required for both acft to fly AP/AT off, and is much more involved.

Hope this helps,
"Never trust a clean Crew Chief"

RE: In Air Refueling And Turbulence

Tue Oct 22, 2002 7:30 am

To keep on the speed, while fueling an aircraft, like Chdmcmanus wrote, we did keep an eye on the INS to keep acceleration/deceleration to "0"... as we got lighter, and reduce power accordingly...
To do very fine tune on the power, I handled only 1 pair of engines (the #2-3) in the KC-135, so as to not "overcorrecting"... The guys in the KC-10 probably do it with the #2 engine only...
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RE: In Air Refueling And Turbulence

Tue Oct 22, 2002 9:44 am

Wow, if that is the worst that has occurred as far as in flight refueling goes, then I am glad!!

Good thing everyone was still safe!

I don't hear much about casualties from in flight refueling. So that is good.

But does it occur at all?
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RE: In Air Refueling And Turbulence

Wed Oct 23, 2002 6:42 am

When you say 'casualties', I assume that you're referring to mid-air collisions between tanker and receiving aircraft. Well, not as far as I know, but our friends from across The Pond may know more.

The only mishaps that I'm aware of (apart from the video mentioned earlier) involve bits of the refuelling apparatus not working or breaking. An additional hazard is one to people on the ground being hit by things falling off aircraft during refuelling. This is not nearly as uncommon as you'd like to think (the 'bits falling off' bit - not the 'people being hit' bit).

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RE: In Air Refueling And Turbulence

Wed Oct 23, 2002 7:52 am

In preparation for the Kosovo campaign some years back some Royal Danish Air Force F-16's practiced air refueling at night over Denmark with a USAF KC-135. Something never tried before in Denmark.

One time they hit slight turbulence and both planes got slightly bent when colliding, but luckily both planes landed safely.

There was some slight "turbulence" on the ground too - in the press - since some people would prohibit air refueling practice flights over populated land.

In the end the Danish F-16 pilots must have learned it. At least I heard of no major incidents in Kosovo even if our F-16's sometimes flew 6 hours long sorties with several refuelings over the Adriatic Sea.

Regards, Preben Norholm
Always keep your number of landings equal to your number of take-offs

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