The South African Air Force (SAAF) refuses to officially acknowledge it, but the Gripens it operates are equipped with a unique gravity refuelling capability that allows them to operate from airfields that lack fuel trucks or fuel pumps and to even be refuelled from drums if necessary.

Its presence can be verified by a visual examination of the SAAF’s aircraft. There are three discrete circles placed flush along the upper right fuselage, between the canards and the tail. A closer look reveals a small four-pointed star symbol, and the wording ‘NATO Code No. F-34’, alongside each. The star is the NATO-standard symbol for a refuelling port, while ‘F-34’ is an instruction that the aircraft uses JP-8, the military equivalent of Jet A-1 aviation fuel.

The circles are the caps of the gravity refuelling ports, each connected directly into one of the Gripen’s three main fuselage tanks, Tank 1, Tank 2 and Tank 3 respectively, as a backup to the aircraft’s standard pressurised refuelling port.

In this contrast-enhanced photograph the three gravity refuelling ports, marked with a four pointed star, are clearly visible. (ADR/Darren Olivier)

In this contrast-enhanced photograph the three gravity refuelling ports, each marked with a four pointed star, are clearly visible. (ADR/Darren Olivier)

None of the other Gripens in service around the world have those ports, all feature smooth metal in the same locations. This is true not only for the Swedish-spec Gripens operated by Sweden, Hungary, and the Czech Republic, but also the export-spec models operated by Thailand.

A close-up view of one of the gravity refuelling ports. Note the lettering indicating JP-8 fuel. (ADR/Darren Olivier)

A close-up view of one of the gravity refuelling ports. Although the ‘F-34’ code indicates JP-8 fuel, the Gripen can operate equally well on the similar Jet A-1 commercial fuel. (ADR/Darren Olivier)

The regular Gripen refuelling port, near the right-hand air intake, is a standard receptacle that requires a pressurised feed to function, so either a fuel truck or a ground fuel pump must be present. This means that SAAF Gripens have the unique ability to be refuelled at any location that lacks a pressure fuelling system, or even to be refuelled from fuel drums if necessary, allowing them to operate either from austere locations or completely independently of airport resources.

The specific reason for including this requirement in the Project Ukhozi specification, under which the Gripens were acquired, has not been made public and the SAAF is not willing to elaborate. However, an examination of SAAF doctrine and tactics implies that the original intention may have been to allow for dispersed operation into either deep rural areas, or semi-prepared airstrips close to the front line, if the country was to go to war.

In practice though it’s rare for any fighter aircraft to deploy to a location that does not have a pressure fuelling system, as nearly all airports have fuel trucks or ground pumps with that capability and the SAAF’s Gripens are in any case always deployed alongside a small technical team, equipped with a Sprinter van and custom-designed trailer from Desert Wolf, that includes all the necessary ground support equipment. In fact, despite the SAAF having deployed its Gripens to airfields and airports all over the country over the years, including a rapid combat deployment to Ndola, Zambia and Kinshasa, DRC in 2013, it has not yet had any need to use the gravity refuelling system.

The main reason for this is that gravity refuelling has two major disadvantages over the standard pressure refuelling approach: It is a much slower process and it can’t be performed while the aircraft is running and the fuel system pressurised.

With the regular single-point pressure refuelling system and a suitable fuel pump, the Gripen can have all of its internal tanks and three drop tanks refilled within an impressive ten minutes, at a rate of around 600+ litres per minute. The onboard systems automatically handle the rerouting of the fuel into each tank in turn. Gravity refuelling, on the hand, is limited by the rate that fuel can be moved from its source without being boosted. If using drums and regularly-sized flexible hoses, filling up the three main centre tanks alone might take over an hour.

It’s also unclear whether the Gripen’s systems support refilling the wing tanks and drop tanks from gravity refuelling, as it would likely require the transfer pump next to the Forward Refuelling Transfer Unit to be used and the system would be unpressurised.

To understand why these limitations exist, it’s important to first describe in brief how the Gripen fuel system works.

The location of the main fuel tanks on the Gripen C. The VT and NGT are excluded.

The location of the main fuel tanks on the Gripen C. The VT and NGT are not shown.

The Gripen C has 11 fuel tanks: Tank 2 Fore (not present in the Gripen D), Tank 2 Aft, Tank 1 Fore, Tank 1 Aft, Tank 3, the Vent Tank, and the Negative G Tank all exist in the fuselage, while each wing has two tanks, Tank 4 and Tank 5. The included diagram shows the relative locations of the tanks, though for the sake of illustration it ignores the Vent Tank and Negative G Tank.

Fighter aircraft fuel systems are highly-complex and designed with multiple failsafes because they have to reliably feed a huge volume of fuel to jet engines in any possible stage of flight, whether the aircraft is flying straight and level, pulling up to +9 g or -3 g, or inverted. The Gripen system uses a combination of a collector tank, boost and jet pumps, and pressurisation to solve the problem.

The collector tank consists of Tank 1 A, Tank 1 F, and the Negative G Tank below both acting in concert and is the only one from which fuel is taken for the engine. A high-power boost pump sits inside the Negative G Tank, which in turn communicates with Tank 1 A/F via a one-way feed. When the aircraft is straight and level or under positive g, the Negative G Tank effectively forms the bottom of Tank 1 A/F so it’s as though the pump is pulling from Tank 1 A/F. When the aircraft is flying inverted or under negative g conditions, the one-way feed of the Negative G Tank ensures that fuel is kept inside it even as the surrounding fuel in Tank 1 A/F is pushed away. However, as the Negative G Tank only stores a limited quantity of fuel, this limits how long the aircraft can fly inverted or while under negative g.

As Tank 1 A/F and the Negative G Tank are the only ones that the engine draws fuel from, the fuel system keeps them topped up by transferring fuel from the other tanks. This is done via the Forward Refuelling Transfer Unit and Aft Refuelling Transfer Unit, a series of small jet pumps, and the main transfer pump, along with the assistance provided by pressurising the other tanks.

The order in which the tanks are transferred into Tank 1 A/F is the following: Drop tanks first (left and right together, then centre), then the combined Tank 2 A/F tank down to 200 kg, then the wing tanks, and finally Tank 3 and the remainder of Tank 2 A/F. As mentioned earlier, Tank 2 F does not exist on the two-seater Gripen D, as the space is taken up by the second seat, but the same process applies. When under high g loads and certain flight angles, the order changes, and the drop tanks are not emptied until the aircraft returns to normal flight.

For the fuel tank pressurisation system a powerful compressor and an intricate web of piping takes bleed air from the engine or APU, runs it past a heat exchanger to cool it down, and pumps it into the fuel tanks. The system is capable of pressurising the tanks to a high enough level that they can self-feed into the transfer pipes, providing a backup in case the jet pumps or transfer pump fail. In normal operations all tanks except for Tank 1 A/F and the Negative G Tank are pressurised, in order to help with fuel transfer, but the onboard computer can adjust the levels as needed to cope with different g loads. Importantly, the aircraft is capable of pressurising tanks during refuelling, helping to speed up the transfer from the refuelling port into the various tanks.

Given all this complexity, designing and testing a fuel system on a fighter aircraft like the Gripen is an expensive, long, and painstaking process. It’s therefore notable that Saab was willing to comply with the SAAF’s requirement to add gravity refuelling as an option, even though it’s a niche capability.

What’s more, the gravity refuelling system on the SAAF Gripens is just one of the more than a dozen changes and modifications that distinguish South Africa’s fleet from that of other Gripen operators. These include South African components such as the ACR500 radio (equipped with the Link-ZA datalink), the GUS-1000 audio management system, and a custom identification friend or foe (IFF) transponder, as well as broad changes to the navigation system, avionics symbology, mission planning tools, and a customised electronic warfare system which appears to have additional support from the on-board fluid cooling circuit. As the accompanying diagram shows, this circuit cools the radar, the avionics bay behind the cockpit, and the electronic warfare jammer installed in the Fin Pod Unit at the top of the tail fin.

A simplified diagram of the cooling circuit in the Gripen, which uses fuel and ram air to cool an oil mixture.

A simplified diagram of the cooling circuit in the Gripen, which uses fuel and ram air to cool an polyalphaolefin fluid called Kylvätska 039 via heat exchangers.

All these changes are possible as a result of the South African Air Force both joining the Gripen programme while the final details of what was then called the Export Baseline Standard were being defined and having a very clear idea via its comprehensive User Requirement Specification of what capabilities it wanted its new fighter to have. This is an important factor, because it’s doubtful that similar alterations would’ve been accommodated in an in-production aircraft for an order as small as the SAAF’s. It’s to be hoped that the SAAF and SANDF might before long become less reticent about the unique systems on board their Gripens and, within reason, inform the South African public the impact that they have.

Because despite the limitations of the gravity refuelling option, it gives South African military commanders one more useful tool on the battlefield which may one day make the difference between a mission succeeding or failing.

Note: ADR reached out to the South African National Defence Force for comment and information regarding this and other modifications to the local spec Gripen, but they responded that all such details are classified. Notwithstanding the right of media outlets to publish classified information when in the public interest, care has been taken with this article to include only open source information.

ADR will also be publishing an in-depth look at the Gripen’s on-board fuel, air, and cooling systems in a future article.

Featured image: A Gripen C of the South African Air Force gets airborne in a darkening sky. ADR/Darren Olivier