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Refueling in Air: Not as Easy as It Looks
Tanker-configured A-7E Corsair with Buddy Store.
If you woke up yesterday wondering about Aerial Refueling and why military pilots do it, or if you were in a bar and someone suddenly mentioned that same question but you thought they were talking about the sans-serif font Arial, well this post is for you!
Jet engines are thirsty. In aircraft with relatively short ranges, fuel quantity roughly determines the time remaining before the engine flames out so the fuel gauge is one of the most closely monitored in the cockpit. (Yes, altitude and airspeed are actually more important, but only until you run out of “go” juice.)
Buddy Store, inflight refueling package.
At high altitude (30,000 feet) and cruising speed, the A-7E’s TF-41 engine consumed about 3,000-lbs of fuel per hour. That’s 50-lbs/minute. At low altitude and full throttle, it burned closer to 7.5K lbs/hr, which is 125-lbs/minute. (18.4 gal/minute!) Big difference.
There’s a lot to keep you busy in a single-pilot jet like the A-7 because there’s nobody else to help with the housekeeping. When flying off the carrier you are constantly doing mental “what-if” calculations to ensure that the current remaining fuel and expected consumption don’t take you below your required minimum fuel for landing.
The time required between launch and recovery is fixed – either 75 minutes (first and last launches of the day) or 105 minutes for the others. Your jet’s fuel load is fixed unless you can get extra fuel from one of the airborne jets with a “Buddy Store”, (the tank with the retracting hose – see above) assuming they haven’t already unloaded their extra fuel. (If you haven’t already seen it, this great video shows the difference between Navy and Air Force refueling equipment.)
A-7E Corsair with 300-gallon drop tank.
Between startup, taxi, launch and climb to altitude, an A-7 would convert about 2,000-lbs of fuel into noise, altitude, and airspeed. The Corsair held 10,000-lbs of fuel internally. When flying off the carrier it usually carried a single 300-gallon drop tank on the inboard wing pylon with 2,000 lbs of fuel to increase its range and loiter time. So, assuming a typical take-off and rendezvous with your wingman, you would have about 9-10,000 lbs of fuel remaining. On the longer launch cycle (105 minutes), you could have 90 minutes remaining before the recovery. One of the requirements is that you land with as close to your maximum landing weight as possible and your “fuel remaining” is the only variable you control.
Fun With Numbers
So let’s do the numbers for a typical carrier-deployed A-7 with a drop tank (200 lbs empty), a Sidewinder missile (180 lbs), and a six-station MER** (300 lbs).
**(MER: multiple ejector rack – a device that attaches to a wing pylon that carries up to 6 general-purpose (unguided) bombs like the MK81 (250 lbs), MK82 (500 lbs), MK83 (1K lbs) and MK84 (2K lbs). Total capacity is only 5K lbs so you could only carry (5) MK83s.).
The aircraft weighs a little under 20K without ammo but with full ammo, (1,000 rounds = approx. 200 lbs) we could use 20K for our empty dry (unfueled) weight. After adding 700 lbs for the external racks, missile and fuel tank, I would use 21K as my flameout (no fuel) weight.
The Corsair’s maximum carrier landing weight was 25.3K lbs. That leaves 4,300 lbs of fuel remaining when you rolled into the groove and called the ball. During daytime operations “folks” became concerned if you called your fuel-remaining below 2.5K because that meant less than a half hour to flame out. The A-7 Low Fuel Light came on with 1,500 lbs of fuel remaining (about 20 minutes maximum if a climb to higher altitude wasn’t needed). And that’s where the aerial refueling comes in and why it’s so important to be able to do it day or night and in all weather conditions.
Most of the time, the aircraft carrier operates out of range of land airfields – at least farther than a typical jet could comfortably divert under low fuel conditions. The nearest land airfield might be in a hostile country and thus inaccessible. Therefore the only response for a low-fuel state is to join the tanker aircraft orbiting over the carrier and get more gas, assuming the fighters haven’t already drained it! (We had very thirsty F-4 Phantoms on my first couple deployments!)
When you’re low on fuel, you need to perform the rendezvous and refueling quickly to avoid the flameout and subsequent ejection and loss of aircraft. Another factor is that the ship can’t complete the recovery and begin moving planes around for the next launch, until the last aircraft (you) has landed. No pressure! Surprisingly the Corsair could actually land with one of the larger fuel buffers, compared to the F-4 Phantom, the EA-6B Prowler, and some of the other planes in the Air Wing.
E-2D Hawkeye refueling from MQ25 drone.
In my time, the E-2C Hawkeye (airborne radar plane) didn’t have a refueling probe and wasn’t able to refuel airborne. Today, the new E-2D Hawkeye does have that capability. In fact, here’s a picture of one refueling from an MQ-25 drone.
I recall the Prowler pilots talking about their limited maximum landing weight fuel due to the weight of all the external electronic jamming pods they carried. It meant they might only get two landing attempts during the day before having to refuel.
These consumption numbers seem astronomical in comparison to other vehicles you’re accustomed to. An A-7 burned up to 7,500-lbs/hr of JP5 fuel (weight per gallon: 6.8 lbs) and that’s 1,137 gallons per hour! That can generate an impressive 500 knots (nautical miles per hour) airspeed (= 575 mph); which equates to an economical 0.5 nm/gallon! That sounds crazy but only if you don’t consider even larger aircraft. A large Air Force cargo aircraft (C‑17 Globemaster III) at high altitude cruise, burns around 5,000-lbs/hr for each of the four engines – 20,000 lbs/hr. total! (That’s 50 gallons/minute total; or 333 lbs/minute.) Care to guess its fuel economy? (For fellow numbers nerds, I’m using JP5; the jet fuel used on ships due to its low flammability. Calculation: 20,000 lbs/6.8 lbs/gal = 2,941.2 gallons/hr. At its customary 450 nm/hr that yields just 0.15 nm/gallon! That’s comparable to a commercial airliner and justified by the number of pounds of cargo it can carry.
Back to the process of inflight refueling…
It is not a risk-free process. That’s one reason that practice and proficiency are so important. The riskiest refueling operations are those performed by those still learning or in bad weather with significant turbulence.
Imagine the damage that the heavy all-metal receiver basket on the end of the hose could do to a receiving aircraft. The basket is flexible in order to collapse as the hose fully retracts back into the tanker package. But that means it has many parts and in a collision, those parts can dislodge or break off and be swallowed by nearby jet intakes, damaging the engines.
One memorable incident involved an A-7 approaching too fast and overshooting and missing the basket while going slightly high. The basket disappeared down the intake! This immediately disrupted the engine airflow which in turn caused loud and flight-suit-soiling compressor stalls which reduced the available thrust dramatically and produced the fortunate result that the A-7 lost some airspeed and backed away from the tanker, causing the hose to be pulled back out of the intake, thus restoring normal power to the engine.
F/A-18 Hornet getting gas.
The hose-swallowing A-7 didn’t appear damaged but as required the shaken pilot declared an Emergency and immediately returned to the ship in case there was undetected damage that could cause imminent engine failure. In this particular incident, the inexperienced pilot and his jet escaped harm. His story quickly circulated through the other squadrons and many young pilots learned again why “Not” to approach the tanker too fast.
You must approach the basket as smoothly as possible. The most common new-guy mistake is over-controlling; chasing the basket as it bounces around. But your much-heavier jet isn’t as nimble so your corrections get ever larger until you either back out to try again (the right response) or you chase it with increasing frustration until something bad happens – a basket or hose slap. The harder you try the worse it gets. The expression for the action you’re trying to avoid is “Killing snakes in the cockpit”. In other words, you’re moving the stick way too much. You have to back away, let the basket settle down, and try again. At night in bad weather with your low fuel light ON, you can imagine that it’s even harder.
The Navy was smart in how it handled safety incidents. It didn’t discourage pilots from reporting problems and mistakes. Everyone reported their mistakes and accepted the consequences knowing that they might be saving someone else’s life by teaching them a second-hand lesson rather than them repeating the same mistake themselves. In an imperfect world, this attitude and process worked remarkably well most of the time.
A “basket slap” was what occurred if the receiving aircraft was out of position or not flying smoothly. The basket could smack the nose or canopy and potentially damage both depending on the severity of the slap. The pitot-static tubes that drive the airspeed and altimeter gauges are usually on the outside of the aircraft’s nose and if damaged, you have a big problem, especially if you need to fly through instrument conditions (without a functioning altimeter or airspeed gauge). Canopy damage was fortunately rare.
Another rare occurrence is the failure of the hose take-up reel on the tanker. A small propeller on the front of the “buddy store” is driven by the slipstream and powers the hydraulics used to take up slack during refueling and retract the hose upon completion. If the hydraulics fail you have two problems. First, when the next aircraft plugs in with the normal slight closure rate (which ensures the proper coupling of the probe and basket), instead of the hose retracting slightly and remaining straight, a sine wave (“s” curve) develops, starting at the receiving aircraft, going up to the tanker store and reflecting back down the hose. When the reflected wave hits the probe on the receiving aircraft it can damage the probe – in severe (though rare) cases actually breaking it off.
The second problem is that the tanker hose can’t be retracted. That’s why the buddy store has an internal hose guillotine that can be activated by the pilot, severing the hose and allowing it to fall free (hopefully not into someone’s backyard).
I did hear of a case where the guillotine failed and the tanker had to land with the hose dangling behind. This creates a significant FOD and flying debris hazard when the basket and hose hit the flight deck. So that was a critical item for the refueling aircraft to notice when plugging in. If the hose reel didn’t take up the slack, you immediately aborted the plug and backed out before the sine wave made it back down the hose. This phenomenon was often the first indication of the buddy store failure.
When your probe engaged the basket, you needed to push the hose back into the buddy store to the “refueling range”, marked by tape on the hose. The hose had to be under compression and retracted to this point to enable the fuel pump that transferred the gas to the receiver.
If a tanker had to land with an extended hose, they were landed last. As they rolled into the groove and called the ball, everyone working on the deck was warned over the loudspeakers and radios so they could duck behind something. The reason you landed the tanker last was because afterward flight operations had to be halted until the flight deck had been walked from bow to stern (all flight deck personnel formed a line abreast at arms’ length) so any debris could be picked up and removed to avoid FOD damage to the jet engines. For the tanker aircraft itself, the pilot would be ready to pull his engines back to idle and then to OFF as soon as the arrestment was confirmed by the deceleration and flight deck crewman using the hand signal to “secure his engines”. This was to avoid the tanker ingesting any debris from the hose and basket. The flight deck crew would be ready with a tow tractor to hook up and move the aircraft out of the landing area. (This is another reason he was landed last.)
Hornet wearing a refueling basket.
The last refueling mishap I remember involved a jet having to land with a basket and about 10 feet of hose still stuck on his probe. Somehow the hose had failed or the buddy store guillotine had activated while the hose was still attached to the jet’s probe.
Most of the time it’s just the basket at the end of the hose that breaks off and remains on the receiver jet’s refueling probe. And based on my Google search of aviation refueling accidents, that has happened more than a few times. If that happens, the jet with the new “basket accessory” would likely fly a straight-in approach, more like an instrument approach than the standard daytime landing pattern.
CH-53 Sea Stallion refueling from KC135.
My final aerial-refueling item has to do with helos. This picture shows a CH53 Sea Stallion doing inflight refueling. Notice how the helo is tilted forward? That’s because it’s going fast (for a helo). Anything I said before about how dangerous aerial refueling is for fixed-wing jets, pales in comparison to the challenges for a helicopter.
The photo is from this short video in which a CH53 refueling from a KC135 suddenly pitches up in turbulence and the helo blades actually cut off the end of its own refueling boom. It must have damaged the blades but apparently insufficiently to cause a crash because according to everything I could find, the helo landed safely (the probe-end landed in a farmer’s yard!).
Finally, if you’re a glutton for punishment, this longer video shows 15 minutes of additional aerial refueling mishaps from the earliest days (in the 60s) up to the present.
(Please send any suggested corrections to my mailbox. Thanks!)
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Great story with two questions. Recall the F-4s really sucked down the fuel faster. How long could they stay in the air compared to an A-7? Assume fuel efficiency improved when the F-14s took over. Did that happen? Remember stories of the pilots getting evaluated after each landing. An “ok” was like an A plus. The highest grade.
Cool. Is that a problem here? It was best diagram I coild find.
I’m told that in full afterburner, a Phantom could burn through all its internal fuel in 5 minutes and it would be supersonic most of that time. In fact it was faster than the Tomcat. Sadly the engine originally planned for the Tomcat never made it through development so it ended up with a lower-powered existing engine. (I welcome comments from anybody out there with more than heresay knowledge of this.) The Tomcat was built around its Phoenix air-to-air missile system – as a superb launching platform that could carry and launch six of those long range missiles. Best of all, they were one of the first fire-and-forget systems. The launching aircraft did not need to continue tracking the target after the missile was launched. That is different than the shorter range Aim7 Sparrow carried by the Phantom.
Man, I’m getting a headache just imagining what ammo and missiles do to weight-and-balance calculations. Or are armaments positioned so that no matter how much you shoot, center of gravity hardly moves?
I could also get a headache imagining what it’s like to fly a helicopter that close to an airplane. But I’ll watch that video later!
No wonder John Kerry is concerned about the carbon footprint of the Ukraine war!
Yes they do change the cg a bit but never sufficiently on the smaller jets to cause problems. The loading of the large cargo jets is another matter. If the cargo shifts aft on climbout, it can cause a crash. And has. I suspect that the Phoenix missiles had to be fired in a specific order to avoid moving the cg too far aft.
They would rather land with the buddy store dragging its hose instead of jettisoning the buddy store altogether?
Psst. It’s actually sans serif, as in sans – without – serifs, the little bits at the ends of letters.
What I linked was the original image that you uploaded. By default Ricochet inserts a downsized version. When you upload an image there is a little menu in the lower right (at least on my Firefox browser) that looks something like this:
It looks like you had “left” alignment. Nothing wrong with that. In the “Link To” menu I almost always choose “Media File” rather than “None.” That makes the image clickable so someone can click on it and see the full-sized image. Under size, you usually have these choices: “Thumbnail,” “Medium,”, “Large,” and “Full Size.” These days the default seems to be “Medium.” I often select “Large,” but it depends on how I want the image to display.
If I had uploaded an image with the Medium Size but neglected to select the “Link to Media File” option, then if I were to go to the post or comment and right-click on the image to “View image,” what I would see displayed is a URL with the suffix “-300×194.jpg”. I then delete the “-300×194” part and get to see the full image in all its 3000×1941 pixel glory (in the case of the image from for which this screenshot was taken).
Sometimes that is necessary in order to make an image legible. In the case of your image, it was readable enough just by zooming in. But out of habit I went through this routine, and then shared.
Of course, none of this is convenient enough to be worth doing on a phone. But on a desktop browser that’s how I view an image in its full uploaded size in cases where the post or comment author didn’t use the “Link to Media File” option. Which is about 99 percent of the cases, it seems.
Jeez KE. Can’t you let up a little on us Navy guys? He was Naval Academy so only got the computer stuff later. I was an NROTC stiff and accounting major and never learned it.
“Sans” is French. If you spell it without the final ‘s’, you’re wrong. If you say it with the final ‘s’, you’re wrong.
Silly language.
Wow, learning a lot
Good catch Ken. Nothing gets past you! :-) (A part of my brain knew this…)
Got it. Thanks. This is actually a “SnipIt” screen capture from my laptop. I didn’t initially understand what you meant by ‘clickable’. I thought you meant it was a hyperlink. I now understand. Thanks for the quick lesson.
You see, that was the part I knew but failed to execute. This is the difficulty of editing one’s own writing…
That does seem counterintuitive doesn’t it? I can no longer identify the source of this memory. This was peacetime and buddy stores were a critical Air Wing asset. That could explain the inhibition to jettison it. There might have been a different policy during Nam. Peacetime navies tend to be frugal. At least in their context. I admit you have a good point…
I was surprised to find out you weren’t dropping drop tanks. Isn’t that why they call them drop tanks?
Peacetime Navy again Judge. If we were going into combat and needed all the speed we could get, they would have been jettisoned (dropped). But in peacetime you hang onto your assets, especially those difficult to replace. Assuming we were launching 60-70 jets a day, we’d have needed a supply chain of ships just carrying new drop tanks, keeping us in business. Not practical. As it was, we did have a tanker pull alongside every week with more jet fuel and probably to top off the ship’s oil (that’s what the Independence CV62 used for the boilers which powered the ship, catapults, etc.). It’s quite a supply operation and the US has had more years of practice doing this than almost anyone.
Thanks for letting me clarify that!
I guess I was also surprised to hear you needed those for patrol style missions. I think of needing those for when the enemy base is outside of the plane’s normal range.
Half of a heavy load from a different angle
Its funny you should mention this. Because airborne refueling is incredibly difficult.
Flying in the turbulent wake of a large aircraft – you just glide in and grab the nozzle?
I think that if it looks easy, speaks to the professionalism, training and quality of the pilots.
Great picture JRo. Let’s see if it’s a legal load. If they matched it on the other side (without the fuel tank)… (…doing quick calculations…)
Nope. Assuming those are Mk 82s (500-lb each), this aircraft would be over the 38K maximum carrier takeoff weight limit if the right wing had the same load. They may have taken some poetic license for a static display aircraft like this one appears to be (otherwise, you were standing too close when it landed!!). Using the 21K-lbs dry weight with a tank and nothing else, you only have another 21K of useful load – fuel plus ordnance and hardware. Subtract 12K of fuel and that leaves 9K for hardware and bombs. Assuming my back-of-the-envelope calcs are correct… Now, if those are the smaller Mk81s (250-lbs each), the load shown would work. (Yes – it is amazing that it could carrier its own weight in fuel and “stuff”.)
BTW: Taking off with at Maximum Takeoff weight requires a helluva hard catapult shot (or an extremely long runway). I flew a tanker-configured A-7 several times (approx. weight 37K lbs.) and it really rattled your teeth getting to the 220+ knots airspeed required by the end of the catapult. Then you had to work hard to accelerate and climb, especially in warm, humid air.
By “work hard” I mean that you flew with a delicate touch and high pucker-factor waiting for the airspeed to accelerate and the rate-of-climb indicator to slowly tick positive, confirming that you were getting farther rather than closer, to the water. As you accelerated, the A-7 would actually gain thrust as more and more air was jammed down the 30-foot deep intake (the front of the engine is roughly even with the back of the wing!). So the faster you went, the more power you had to climb and accelerate. But that first several seconds were always dicey and that’s why only experienced pilots (with at least one deployment of experience) normally flew those. (I was an unplanned exception to this rule. https://ricochet.com/1010482/first-flight-with-the-skipper/)
You’re right about the training! That’s the key determinant of success or failure and that’s one of the ways a military suffers when its budgets are restricted for various reasons. The budget must be appropriate for the size or it will only look good on paper. (IMHO)> That’s not an argument for unlimited military spending. Only that we spend it where it counts and has the most bang for its buck. This seems to be a lesson that must be relearned every few years… Talk to any active duty military pilot – especially the youngest and least experienced – and they will complain that they don’t “fly enough”.
That was the case in my time also and I suspect even in my grandfather’s day, back in the 1920s, flying Curtis JN4s and 6s. (I actually have a post about him if you’re curious…https://ricochet.com/983413/my-grandfather-and-the-fort-snelling-bridge/)
How did I miss that one?
Yes. I think this is why the Russians are having so much trouble in Ukraine. They’re equipment isnt properly maintained, and their troops are under trained, under exercised. (I mean by military exercised – not personal physical conditioning)
This is possibly a by product of corruption. They buy x tanks there are x tanks in the parking lot… but If they buy X or Super X+ fuel – how much will be in the fuel tanks of the trucks and tanks, and not sold off in the black market? That’s more difficult to track.
Also, if they can buy Chinese tires that cost less and put the rest in their pockets, thinking it doesn’t matter until the vehicles actually get sent somewhere…
Actually that is only part of the problem, before deployment the trucks hadnt been run in months. The tires deteriorate in sunlight. Most trucks like this are fitted with a variable tire pressure system – so that when they get stuck in the mud – they can reduce the tire pressure, that increases the surface area of the tire in contact with the ground, and increases traction – to get them unstuck from the mud. But because these tires have been sitting for so long, the compromised sidewalls of the tire tears apart instead.
There is an interview with a military equipment procurement specialist – who examined photos of the disabled trucks, and was talking about that…
Found it:
They should have gotten the APC from Alien 2/Aliens, no tires! And no treads either.