The Gripen C and D operated by the South African Air Force (SAAF) can, in a pinch, do something few other contemporary fighter aircraft are capable of: refuel at an austere airfield using nothing more than standard fuel drums.
This is because, in addition to the industry-standard single-point pressure refuelling system on the aircraft, the SAAF Gripens include direct gravity refuelling ports above the main fuselage fuel tanks. So the aircraft can be refuelled using non-specialised hoses directly from drums or bladders, even if there are no fuel pumps available.
This modification is unique to the SAAF’s Gripens, as it was specifically requested in the Supply Terms negotiated with Saab for Project Ukhozi. None of the other Gripens in service elsewhere in the world have direct-feed ports; all follow the regular approach of relying entirely on the single point pressure refuelling system.
It’s an odd requirement. Direct-feed gravity refuelling is an archaic practice that was swiftly abandoned on fighter aircraft once single-point pressure refuelling became viable in the 1950s, and by the late 1960s it remained on only a few fighters as an emergency fallback. Indeed, few other fighters still have this capability: The F-16, Typhoon, Rafale, MiG-29, and the like all rely entirely on single-point pressure refuelling.
Direct-feed gravity refuelling’s disadvantages compared to the more modern method are numerous: It’s much slower, whereas all the internal tanks and three drop tanks can be refilled on a Gripen in an impressive ten minutes via the regular single-point method, doing the same using direct feed gravity refuelling could take over an hour. Secondly, it’s riskier to the airframe, as it requires ground crew to walk on, and run hoses over, the wing and fuselage. And thirdly, it presents a greater risk of fire, and can’t be done while the aircraft’s engine is operating and the fuel tanks are pressurised.
Why then has the SAAF insisted on having these changes as a requirement, considering that the fuel system is one of the more complex parts of an aircraft?
Unfortunately, when we reached out to the SAAF to request comment and information regarding this and other modifications, their official response was that all such requested changes were classified and could not be revealed. Thus, we can only engage in informed speculation based on an examination of SAAF doctrine and tactics, which point to the SAAF requiring this refuelling option as an emergency backup to allow for wartime dispersed operations into either deep rural areas or semi-prepared airstrips close to the front-line even if fuel pumps are unavailable or break down.
It is, in other words, a last-resort emergency option, meant to provide commanders with one more tool on the battlefield that might one day make the difference between mission success and failure. It’s a reflection of a key principle in long-held South African National Defence Force doctrine, which is to use high-technology solutions where they make sense as force multipliers, but to maintain low-tech fallbacks where possible to cater for equipment and system failures.
In this regard, it’s a useful addition to the other expeditionary capabilities of the Gripen, an aircraft that was specifically designed to be able to operate from dispersed locations – such as civilian roads – with minimal logistical support and only a small ground crew.
The Gripen requires only a 17 m x 800 m surface to land and take off, can be supported for days using only spares and support equipment that can fit into an Oryx or similar medium helicopter, is designed for quick and easy part changes up to and including the engine, and has a sophisticated self-diagnostic health and usage monitoring system on board. In fact, the standard SAAF ground support for a Gripen deployment is just a pair of Sprinter 519 vans, each equipped with a custom Desert Wolf trailer carrying spare parts, equipment, and amenities for the crew, as part of the Gripen Support Vehicle and Support Trailer (M-TIES) system.
While South Africa doesn’t face the existential threats that Sweden does, which necessitates their need to not be dependent on air bases, this expeditionary capability has been immensely useful each time the aircraft have been deployed away from their home base at AFB Makhado. For the 2010 Football World Cup, 2 Squadron was able to deploy Gripens all around the country with little logistical support, despite only recently being made familiar with the aircraft and its systems. Subsequent deployments for airshows, exercises, and so forth have confirmed this early experience.
The value of this capability was further highlighted in early 2013, when the South African National Defence Force mobilised dozens of aircraft and thousands of troops to prepare a possible response to the attacks on its troops in Bangui, Central African Republic. On extremely short notice – less than 24 hours – four fully-armed and equipped Gripens were flown all the way to Kinshasa in the Democratic Republic of Congo, via a single stop in Ndola, Zambia, before the call came to stand down. Needless to say, this would have been more difficult on the comparatively maintenance-heavy Cheetahs.
In acquiring the Gripens, therefore, the SAAF not only got an aircraft already well-suited to dispersed expeditionary operations, but insisted on adding a change that made it even more supportable when all else around it breaks down. It’s also an indication of how seriously Saab took the contract, as fuel system changes are rarely made for customers, especially when their order does not run to the high dozens or hundreds of aircraft. To understand why this is, it’s important to first understand how the Gripen’s fuel system works.
The Gripen C has five main fuel tanks, split into eleven separate cells with individual purposes, and which cater for the aircraft’s complex internal geometry. Tank 2 is the front-most tank, just behind the cockpit, and is split into Tank 2 Aft and Tank 2 Fore. On the Gripen D Tank 2 Fore has been removed to make space for the second seat. Tank 1, the central fuselage tank, is split into Tank 1 Fore and Tank 1 Aft, and has a Vent Tank and Negative G Tank as specialised cells. Tank 3 is the aft-most tank, and wraps around the engine in part of the rear fuselage. Each wing has two tanks, Tank 4 at the leading edge and Tank 5 behind it. These cross-feed to each other and to the matching tanks on the other wing. The included diagram shows the relative locations of the tanks, though for the sake of illustration it excludes the Vent and Negative G tanks.
In order to reliably feed huge quantities of fuel into the collector tank and engine in all phases of flight, even when the aircraft is inverted or sustaining severe positive or negative g, the fuel system is highly-complex and designed with multiple redundant fail-safes.
Tank 1 and the Negative G Tank collectively form the aircraft’s collector tank, meaning that it’s the only location from which fuel is fed to the engine. The Negative G Tank is located below Tank 1 and is connected via a one-way valve. It contains the powerful boost pump that sends fuel to the engine. In normal straight and level flight, the Negative G Tank is effectively the bottom of Tank 1 and they form a combined cell. However, when the aircraft is inverted or sustaining negative g conditions, the one-way valve prevents fuel from running back into Tank 1 and thus keeps feeding the boost pump. As the Negative G Tank is relatively small, it only carries a limited amount of fuel and so (as on all fighters) the pilot’s operating handbook specifies a maximum time, typically 15-30 seconds, that the plane may fly inverted or sustain negative g.
The other tanks feed into that collector tank in the following order: Drop tanks first (left and right together, then centre), then both cells of Tank 2 down to 200 kg, then Tanks 4 and 5 in the wings, and finally Tank 3 and the remainder of Tank 2 to keep the centre of gravity balanced. When under high g loads or at unusual flight attitudes, the drop tanks are ignored and not used until the aircraft returns to normal flight. The onboard computer is also able to adjust the transfer of fuel in real-time to compensate for unusual conditions and keep the centre of gravity balanced, while the pilot can manually override the standard transfer settings.
Transfers are driven by small jet pumps inside some of the tanks and a main transfer pump, working in concert with the Forward Refuelling Transfer Unit and Aft Refuelling Transfer Unit, and augmented by the pressurisation inside the tanks. That in turn is driven by a powerful compressor and an intricate web of piping that takes bleed air from the engine or Auxiliary Power Unit (APU), runs it past a heat exchanger to cool it down, and pumps it into the fuel tanks. The pressurisation is high enough that even if the transfer and jet pumps fail, the onboard computer can still transfer fuel from the outer tanks to Tank 1 using pressurisation alone, albeit at a lower rate.
All this redundancy matters, because fuel pumps can and do break. Just recently a SAAF Gripen declared an emergency when the main fuel transfer temporarily failed (in a way apparently unrelated to the SAAF fuel mod), but that was far from a unique situation. The Cheetah fleet experienced numerous in-flight emergencies caused by fuel system failures, as did the Mirages before them. In air forces across the world, there have been examples of fighter jets that crashed due to fuel starvation when they had perfectly good fuel in drop tanks or internal fuel tanks, but no way to transfer it to the engine.
Given this, it should be clear why changes to a fighter jet’s fuel system are not made lightly or without significant thought and testing. That Saab was willing to meet the SAAF’s request in this case is a reflection of the good timing of the SAAF’s Gripen acquisition, which came when the C/D variant’s final baseline was still being developed and there was scope for changes and adaptations to suit the SAAF’s unique requirements. This fuel mod was just one of dozens of SAAF-specific changes, including local components like the ACR500 radio (equipped with the Link ZA data link), a custom identification friend-or-foe system from Tellumat, and broad changes to the navigation system, avionics symbology, mission planning tools and the onboard electronic warfare system. For the latter system, subtle details on the Wing Tip Unit antennas that contain ESM receivers and the Fin Pod Unit that holds one of the on-board jammers that differ from those on other Gripens can be seen.
In sum, the SAAF acquired a very capable aircraft in the Gripen and, as a result of good timing, a well-researched and detailed specification, and the close co-operation of Saab, managed to make it even better for its own needs and environment. Amidst the controversy around the way the Gripen acquisition, and the rest of the Arms Deal, was run at a political level, we should not forget that at the lowest level, the acquisition processes inside the SAAF and Armscor worked well and closely matched the SAAF’s needs.