JonathanRP
Topic Author
Posts: 53
Joined: Tue Sep 20, 2016 7:38 pm

Air-to-air refuelling; what about wake turbulence?

Sun Apr 09, 2017 8:02 pm

Hi all,

just a quick question about something that's been bothering me for a while! When aircraft are being refuelled mid-air do they not encounter wake turbulence? The separations required on approaches are well documented, but when aircraft are flying much closer together, doesn't this cause issues?
 
greg85
Posts: 83
Joined: Tue Jun 26, 2007 9:45 am

Re: Air-to-air refuelling; what about wake turbulence?

Mon Apr 10, 2017 2:48 pm

I've never been a military pilot. But I've spoken with several of my colleagues, who flew jets such as the f-4 Phantom, Tornado, Jaguar and Mirage 2000. And yes, wake turbulence is an issue. Procedures are designed accordingly to approach and leave the tanker safely. I remember an ex Tornado guy telling me that the "basket" would move up and right just as they are about to put the probe in. It was predictable, so the guys knew to fly up and right at the correct point to capture it. He said it got harder if you refuelled from an American tanker, with the basket coming out of the end of the "flying boom". It was harder because the boom operator would try to "help", and they'd move the basket down and left, just as the pilot moved up and right. Several radio conversations later, the boom driver "kept it f£&@!'g still!".
 
26point2
Posts: 905
Joined: Wed Mar 03, 2010 6:01 am

Re: Air-to-air refuelling; what about wake turbulence?

Mon Apr 10, 2017 3:07 pm

In day-to-day flight operations we are concerned about wake turbulence but really what we are trying to avoid is the wing tip vortices. I suppose air-to-air refueling is at such close tange and between tanker wingtips that wingtip vortices are perhaps not a concern but the actual tubulence left behind by the mass of the aircraft? Is there even a name for this or do we just call anything associated with turbulent air folloiwing an aircraft "wake turbulence"?. An honest question.

It has always puzzled me why ground controllers say "caution, wake turbulence" when instructed to taxi behind a large jet when what they really should say is "caution, jet blast".
 
mmo
Posts: 1014
Joined: Thu Apr 18, 2013 3:04 pm

Re: Air-to-air refuelling; what about wake turbulence?

Mon Apr 10, 2017 6:36 pm

After spending countless times behind a 135 and -10, with the typical plug boom, there is very little turbulence. I flew the B-52 on active duty and then the F-4 and F-15 in the Guard and each was fairly easy to refuel. I would say the B-52 was the most difficult. I will date myself but I flew D and G models, and the D was old fashioned cables and pullies. It was a handful in the best of times and a real pig most other times. However, if you refueled at a slightly faster speed, 280 KIAS, it was much easier. It was not uncommon to have an onload of 60,000lbs from one tanker or 100,000lbs from 2. If you couldn't refuel you were in the wrong business.

In all aircraft as you went from the pre-contact to contact position, you did experience a slight amount of aerodynamic interference. It was nothing you couldn't handle and most times you got so used to it you didn't even notice it.
If we weren't all crazy we'd all go insane!
 
tupperjets
Posts: 13
Joined: Mon Oct 31, 2016 11:41 pm

Re: Air-to-air refuelling; what about wake turbulence?

Tue Apr 11, 2017 6:57 am

I found this article interesting.

The first thing in our "Dash 1" technical orders states: "Because of the magnitude of interrelated aerodynamic effects flying two aircraft at close vertical proximity is unsafe." So basically the USAF tells us that our job is not safe right off the bat. But I've hit turbulence during refueling where we had to initiate an emergency separation or "breakaway" several times. The boom operator disconnects, the tanker hits full throttle and the receiver plane drops a thousand feet in a couple seconds just to get separation. We also practice this all the time. Almost every flight since we have to separate and give up MARSA (Military Assumes Responsibility of the Separation of Aircraft) anyway.
lessonslearned.faa.gov
 
JonathanRP
Topic Author
Posts: 53
Joined: Tue Sep 20, 2016 7:38 pm

Re: Air-to-air refuelling; what about wake turbulence?

Thu Apr 13, 2017 10:28 pm

Thanks a lot for all your responses!
 
GlobalMoose
Posts: 45
Joined: Sun Aug 19, 2012 10:44 pm

Re: Air-to-air refuelling; what about wake turbulence?

Thu Apr 13, 2017 10:46 pm

After just refueling today, wake turbulence isn't really that bad at all, in fact, i'm more worried about atmospheric turbulence than any 'turbies' produced from my tanker. The -135 and -10 each present their own challenges, advantages, and 'turbulence.' Approaching the -135, you'll often feel a 'burble' (rumbling in the aircraft, much like driving fast on a roughly paved road) passing up to the astern position (50 feet in trail). Also, the -135 has many more visual references on the aircraft (antennas, convenient locations of the engines in my window, etc) to help maintain position in both astern and in contact (in addition to the pilot director lights). A downside of the -135 is the relatively small refueling envelope.

The KC-10 is much, much larger. They have a burble that you feel in the aircraft all the way up to contact. Once there, you have a much larger refueling envelope than the -135, however, there are substantially fewer references to maintain your position. Also, while you are supposed to maintain centerline, if you get too far left or too far right, the exhaust from the number 1 & 3 engines tends to bump you back toward centerline, however, never do you have a jolt that one would expect from traditional atmospheric turbulence.
When it absolutely positively has to be there ... at some point.
 
RetiredWeasel
Posts: 460
Joined: Sat Jul 05, 2014 8:16 pm

Re: Air-to-air refuelling; what about wake turbulence?

Thu Apr 13, 2017 10:57 pm

The only turbulence I worried about during refueling was the atmospheric turbulence, CAT and convective type as Moose mentioned. Tanker crews were pretty good at requesting altitudes to avoid it and also to try and stay out of the clouds. Flying the F-4 for 8 years with scores of hookups I cant remember encountering any thing that really shook me up. You knew where the wingtip turbulence was and disturbed air from the tankers engines, and you just avoided it.
 
GalaxyFlyer
Posts: 72
Joined: Fri Jan 01, 2016 4:44 am

Re: Air-to-air refuelling; what about wake turbulence?

Fri Apr 14, 2017 2:07 am

While refueling the C-5 behind the KC-10, the center engine would push the C-5 to a bit of skid off go one side. If you tried to stay perfectly centered on the lights you would pushed either way. Usually, you just state to one side and accepted the yaw.

GF
 
User avatar
tjwgrr
Posts: 2144
Joined: Fri Mar 24, 2000 4:09 am

Re: Air-to-air refuelling; what about wake turbulence?

Fri Apr 21, 2017 2:59 pm

GalaxyFlyer wrote:
While refueling the C-5 behind the KC-10, the center engine would push the C-5 to a bit of skid off go one side. If you tried to stay perfectly centered on the lights you would pushed either way. Usually, you just state to one side and accepted the yaw.

GF


Conversely, I've heard the C-5 generates a bow wake. Can that be felt by, or affect, the tanker?
Direct KNOBS, maintain 2700' until established on the localizer, cleared ILS runway 26 left approach.
 
GlobalMoose
Posts: 45
Joined: Sun Aug 19, 2012 10:44 pm

Re: Air-to-air refuelling; what about wake turbulence?

Sun Apr 23, 2017 10:46 pm

The C-5 and the C-17 (and I'm sure the other heavies do as well) create a bow 'wave' (not wake). If the receiver goes in too quickly or not on the correct 30 degree line, you can create an unsafe situation between the two aircraft.

After talking to the -135 guys, as the -17 goes into and out of contact, their trim wheel spins like crazy (with the autopilot on) adjusting for the unique aerodynamics caused by two large aircraft flying in close proximity. Additionally, if the receiver comes in too quickly, the trim (autopilot) cannot keep up with the change in airflow and kick off, causing the -135 to dive into the receiver if the tanker pilot isn't shadowing the yoke, ready to intervene.

Some aircraft are easier to refuel than others, some can be felt more in the tanker as well. I know the -17 is unique as it is so maneuverable, even during AR, it can cause problems for both sets of pilots.
When it absolutely positively has to be there ... at some point.
 
User avatar
Moose135
Posts: 2668
Joined: Mon Oct 04, 2004 11:27 pm

Re: Air-to-air refuelling; what about wake turbulence?

Mon Apr 24, 2017 12:11 am

tjwgrr wrote:
Conversely, I've heard the C-5 generates a bow wake. Can that be felt by, or affect, the tanker?

Absolutely! In the -135, when a C-5 moved into the contact position, you definitely felt it, and as GlobalMoose said, the elevator trim wheel would be spinning to keep up with it, and at times, we would have to help it along with some movement of the yoke.
KC-135 - Passing gas and taking names!
 
benbeny
Posts: 167
Joined: Thu Nov 03, 2016 1:44 pm

Re: Air-to-air refuelling; what about wake turbulence?

Wed Apr 26, 2017 3:01 pm

I've heard some pilots at the receiver end prefer to offset it a bit left or right for smoother and easier flying. Is it true?
 
RetiredWeasel
Posts: 460
Joined: Sat Jul 05, 2014 8:16 pm

Re: Air-to-air refuelling; what about wake turbulence?

Wed Apr 26, 2017 9:39 pm

benbeny wrote:
I've heard some pilots at the receiver end prefer to offset it a bit left or right for smoother and easier flying. Is it true?

I can't speak for the big iron, but for fighters, no.
Edit: I better add that some of the later fighters or even some earlier ones, may have had an offset refueling receptacle on their aircraft....so they may have adjusted slightly to center the boom. But it wasn't due to turbulence.
 
jarheadk5
Posts: 233
Joined: Thu Nov 03, 2005 2:45 pm

Re: Air-to-air refuelling; what about wake turbulence?

Wed Apr 26, 2017 9:59 pm

tjwgrr wrote:
Conversely, I've heard the C-5 generates a bow wake. Can that be felt by, or affect, the tanker?


Former KC-10 boom operator here.
All the large receivers (KC-10, C-17, C-5, E-4/VC-25 to a greater extent; E-3, E-6, E-8, xC-135, C-32 to a lesser extent), generate a "bow wave". That's the terminology used in the -10 community; I can't speak for the -135 community.
The C-5's bow wave can definitely be felt in the KC-10, even with the autopilot on. Autopilot-off contacts with the C-5 were... interesting. Lots of porpoising as the -10 PF worked the stab trim and yoke to maintain altitude, as the bow wave from the C-5 pushed the -10's tail up as it approached the contact position. Most C-5s I refueled tended to stay a bit off-center; we joked that we could tell which seat a C-5 contact was being flown from by which side of centerline they tended to favor in contact.
The large receiver's bow wave also pushes the boom up as the receiver approaches contact. -10 BOs have to push down on the boom flight control stick to counter this, but don't have to add trim to the boom flight controls. -135 BOs have to add "boom trim" to their boom, due to the nature of the -135 boom's flight control system.


Wake turbulence can be a problem for receivers approaching the tanker from too low. I've experienced it as the receiver while approaching another KC-10 as the tanker, with a new pilot at the controls and an IP who disguised his slow reaction to the noob's poor approach as "a learning experience for the student". When the approach to contact is flown properly, wake turbulence is not a factor.
-Boom stowed, leaving position.
 
benbeny
Posts: 167
Joined: Thu Nov 03, 2016 1:44 pm

Re: Air-to-air refuelling; what about wake turbulence?

Thu Apr 27, 2017 6:30 pm

It's interesting to see how people manage to fly something so big so close to another flying objects. Is it easier to refuel with boom, or drogue system?
 
GlobalMoose
Posts: 45
Joined: Sun Aug 19, 2012 10:44 pm

Re: Air-to-air refuelling; what about wake turbulence?

Fri Apr 28, 2017 12:10 pm

After flying the -17 for 6 years and instructing for the past 2 of those 6, I have never been taught or heard the technique of lining up on one side or the other to help avoid tanker turbulence. If anything, you're always taught to be in the center of the envelope as indicated by the pilot director (direction?) lights (PDI's) or by asking the boom to 'say numbers' as he has indicators of extension, elevation, and azimuth.

To align yourself with center, you are taught to line yourself up with the left set of the PDI's if in the pilot's seat or the right set of PDI's if in the copilot's seat (check the database for a good picture of the belly of a tanker).

With regard to which system is easier, I really don't know as I have only refueled off of a boom. I've been told the boom acts to stabilize the two aircraft when in contact (however I cannot confirm). Also, you can transfer much more fuel through the boom system rather than drogue system - since the AF needed to refuel large tanker/transport/bomber aircraft, and in the quest for commonality, most AF aircraft were fitted for the boom system, including fighter aircraft. Navy and MC aircraft, on the other hand use the drogue system as you can't put a boom on an F/A-18 flying around the carrier pattern to help refuel other fighter aircraft in need.
When it absolutely positively has to be there ... at some point.
 
GalaxyFlyer
Posts: 72
Joined: Fri Jan 01, 2016 4:44 am

Re: Air-to-air refuelling; what about wake turbulence?

Sat Apr 29, 2017 1:49 am

I suspect the -17's rudder is low enough to be out of the -10's center engine's exhaust flow. It wasn't much off center, but a bit.

The probe on the F-100 was on the right side, so a bit odd feeling. Pilots turn left, I guess. The trick was NOT looking at the drogue but just fly formation off the tanker after getting probe lined up, add some thrust and hold position until picking up the drogue. The boomer couldn't do much to help the hook.

GF
 
User avatar
Moose135
Posts: 2668
Joined: Mon Oct 04, 2004 11:27 pm

Re: Air-to-air refuelling; what about wake turbulence?

Sat Apr 29, 2017 5:01 pm

GlobalMoose wrote:
To align yourself with center, you are taught to line yourself up with the left set of the PDI's if in the pilot's seat or the right set of PDI's if in the copilot's seat (check the database for a good picture of the belly of a tanker).


The PDI light housings are the two black lines either side of center line near the nose of the tanker.

Image

Our squadron ops officer flew the Looking Glass EC-135s for a number of years, and did a ton of receiver work. One day on a training sortie, he gave us a demo, lined up behind #1 engine, then moved over behind #4, showing the effects of flying in different areas behind the tanker.

Image
KC-135 - Passing gas and taking names!
 
jarheadk5
Posts: 233
Joined: Thu Nov 03, 2005 2:45 pm

Re: Air-to-air refuelling; what about wake turbulence?

Sat Apr 29, 2017 10:19 pm

benbeny wrote:
Is it easier to refuel with boom, or drogue system?

As a KC-10 boom operator, drogue AR is boring*. Flip a couple switches, push a button before each contact, and make a couple radio calls. The receiver does all the work; the tanker just holds position. KC-135 BOs on a jet with the BDA (Boom Drogue Adaptor, aka the "Iron Maiden") installed have a bit more work to do, since they still have to fly the boom steady, but not as much work as making a normal boom contact. KC-10 WARP & KC-135 MPRS are similar - flip a couple switches, push a button, make a radio call. Again, the receiver is doing all the work. As already noted, boom AR is much faster; the boom's ID is much larger than the drogue hose's ID, so the boom's fuel flow rate is much higher.

* - assuming the system is working correctly. This isn't always the case, unfortunately.



GlobalMoose wrote:
After flying the -17 for 6 years and instructing for the past 2 of those 6, I have never been taught or heard the technique of lining up on one side or the other to help avoid tanker turbulence. If anything, you're always taught to be in the center of the envelope as indicated by the pilot director (direction?) lights (PDI's) or by asking the boom to 'say numbers' as he has indicators of extension, elevation, and azimuth.

I think it's just a FRED thing; I don't recall seeing Barney consistently hanging out a few degrees off-center during an otherwise-stable contact.


EDIT: Nice steamjet pics, Moose135!
-Boom stowed, leaving position.
 
User avatar
Moose135
Posts: 2668
Joined: Mon Oct 04, 2004 11:27 pm

Re: Air-to-air refuelling; what about wake turbulence?

Sun Apr 30, 2017 3:15 am

jarheadk5 wrote:
Nice steamjet pics, Moose135!

Thanks, Boom! I was flying a KC-135A/RT receiver equipped tanker out of Grissom. Good times!
KC-135 - Passing gas and taking names!

Who is online

Users browsing this forum: No registered users and 2 guests

Popular Searches On Airliners.net

Top Photos of Last:   24 Hours  •  48 Hours  •  7 Days  •  30 Days  •  180 Days  •  365 Days  •  All Time

Military Aircraft Every type from fighters to helicopters from air forces around the globe

Classic Airliners Props and jets from the good old days

Flight Decks Views from inside the cockpit

Aircraft Cabins Passenger cabin shots showing seat arrangements as well as cargo aircraft interior

Cargo Aircraft Pictures of great freighter aircraft

Government Aircraft Aircraft flying government officials

Helicopters Our large helicopter section. Both military and civil versions

Blimps / Airships Everything from the Goodyear blimp to the Zeppelin

Night Photos Beautiful shots taken while the sun is below the horizon

Accidents Accident, incident and crash related photos

Air to Air Photos taken by airborne photographers of airborne aircraft

Special Paint Schemes Aircraft painted in beautiful and original liveries

Airport Overviews Airport overviews from the air or ground

Tails and Winglets Tail and Winglet closeups with beautiful airline logos