By Owen Zupp
For the great majority of endeavours, aviation is the domain of symmetry and streamlining, while weight and drag are the constant enemies of efficient flight. And yet on the rare occasion, necessity becomes the mother of an unconventional invention that turns heads and creates conversation. Such is the case when a Boeing 747-400 takes to the skies with an additional engine attached to its wing, albeit a ‘spare’ that does not produce thrust.
The Fifth Pod.
As long as airliners have been powered by jet engines, a common design has called for the mounting of the power plant on the wing. Connected to the airframe by ‘pylons’, the engine is encased in a cylindrical cowling designed to house components, maximise engine performance and minimise any resistance to the airflow. This arrangement is sometimes referred to as a ‘pod’.
Today, the sight of huge turbofan engines beneath the wings has become commonplace, but normal design and operations call for an equal number of engines either side. Commonly known as the ‘Fifth Pod’ it has also been referred to as ‘V Pod’ and ‘Spare Engine Carriage’. The carriage of a spare engine is a practise that dates back decades and for QANTAS can be traced to 1959 with the introduction of the Boeing 707. An early adopter of the procedure, the airline remains one of the few that still transports spare engines by air when the situation dictates.
This occurred recently when a Boeing 747-400 was stranded in Johannesburg, in need of a replacement engine. While the option existed to air freight an engine from a pool held in London, the ability to dispatch a QANTAS-owned engine from Sydney as a fifth-pod on a commercial service was more efficient, both in terms of cost and time. Within hours of the decision being made, the engine was being mounted beneath the wing of VH-OJS, ‘Hamilton Island’.
Fitting the Fifth.
Although an infrequent event, the fifth-pod carriage is one that QANTAS is experienced in conducting. Captain Mike Galvin, Head of Fleet Operations at QANTAS related that such an exercise draws together various elements of Flight Operations including flight planning, flight dispatch, load control and crewing before the flight ever leaves the ground. Administratively, fifth-pod operations fall within the bounds of QANTAS’s Air Operator’s Certificate and do not call for any specific approval to be sought. However, at the very heart of the process is the engineering team, specific components and a detailed procedure.
The engine to be carried is complete except for the removal of the low pressure compressor (fan) blades, a compressor splitter fairing and in some cases, a hydraulic pump prior to flight. The fifth-pod operation is only approved for the carriage of Rolls Royce RB211 engines on Rolls Royce powered Boeing 747-400s of the QANTAS fleet. The GE-powered aircraft are not certified for the task and the GE engines cannot be carried on the Rolls Royce powered 747-400s due to a lack of ground clearance. Even in the case of the RB211, ground clearance must be monitored and care taken as boarding, disembarking and the loading of fuel and freight can constantly alter the position of the aircraft and the spare engine relative to the ground.
Specific equipment and tooling exists to make the fifth-pod possible. The External Spare Engine Carriage Kit (ESECK) includes the mounting strut, fairings, fasteners, air deflector, nose cowl extension and a ‘deflector plug’ as well as a number of supplementary items. Additionally, a shipping container of equipment and tooling as well as two disassembled engine change stands are shipped in support of the fifth-pod.
The mounting strut weighs approximately 700kg and is attached to the underside of the left wing, between the fuselage and the inboard engine. The bolts used to attach the strut to the wing are different to those normally used to attach an engine to the wing, so they are colour-coded to prevent any confusion.
The ‘air deflector’ is more commonly referred to as the ‘doughnut’ due to its similarity in appearance. It is installed over the compressor inlet and both limits ice build-up on the exposed guide vanes and regulates airflow. The ‘deflector plug’ is installed in conjunction with the air deflector to block airflow and prevent rotation of the N2 and N3 rotors within the engine.
The fibreglass nose cowl extension is attached to the engine inlet to minimise drag while the forward fairing attaches to and covers the front end of the strut. The engine’s hinged cowls must be secured prior to mounting as insufficient space exists once the engine is on the wing. All in all, the weight involved in the carriage of the spare engine and its associated fittings is approximately 7,600kg.
Once mounted, the final step of the installation is to activate the ‘5th Engine Carriage’ switch in the Maintenance Equipment Centre (MEC) to modify the inputs to the Air Data Computer (ADC) and Flight Management Computers (FMC) to allow for the performance changes associated with the fifth-pod.
In the case of fitting the spare engine to VH-OJS, Captain Galvin relates that the process began at 7am and the aircraft was towed to the gate just over four hours later. It was an impressive effort by the engineering staff that saw the experienced engineers mentoring the younger generation in a process that QANTAS has utilised since the earliest of days and the 707.
The Fifth Pod in Flight.
As one can imagine, the attachment of an additional, but non-operating engine to one side of an airframe comes with some modification to aircraft performance and procedures. Surprisingly, the penalties are not as significant as one might imagine or as internet gossip may lead one to believe.
In planning, the maximum take-off weight (MTOW) is reduced to maintain the aircraft within centre-of-gravity limits. In flight, the maximum speed technically remains at Mach 0.85, but operations are recommended at Mach 0.78 as a precaution against possible airframe buffeting. On take-off, where thrust is high and weight is at its maximum, there is a need for 1.0 units of right rudder trim to be set, although this demand becomes less as the flight progresses.
Operational speeds at the critical phases of flight are minimally modified. On take-off, V1, Vr and V2 are only increased by 2 to 3 knots, depending on weight while the approach speed is varied by 3 knots when coming into land. Similarly, climb gradients are reduced by approximately one percent enroute and in the event of a go-around.
Delving more deeply into the performance manuals for the Boeing 747-400, at MTOW it will only take one minute more, 3 miles further and burn an additional 200kg of fuel to reach 32,000 feet or Flight Level 320. Once at altitude, the reduced Mach Number of 0.78 for the fifth pod equates to around 30 knots less True Airspeed (TAS) over a normally configured 747 flying at Maximum Range Cruise (MRC) speed of around 0.83 Mach at 340 tonnes.
On the flight deck, the implementation of the performance data is relatively seamless. While the performance figures are readily available in table form, the Boeing 747-400’s Flight Management Computer (FMC) has the facility to select a dedicated section for fifth-pod operations via the Control Display Unit, or CDU. By selecting the prompt titled “SPARE ENG”, the FMC automatically a fifth-pod speed schedule to correspond to its climb at 305knot/0.78M, Long Range Cruise at 0.78M and descent at 0.78M into 290 knots. Additionally, the FMC can provide ’engine out’ climb and drift-down speeds in the event of a failure. The Vref approach speed will also be automatically increased by 3 knots.
With the speed modified to Mach 0.78, the increased flight time dictated that the flight’s routing between Sydney and Johannesburg also be modified. Normally QANTAS Flight 63 (QF63) tracks south overhead Tasmania as it plots the shortest and very southern route to Africa. For the fifth-pod flight, QF63 was staged through Perth to enable a change of crew in the Western Australian capital. This added additional time to the flight but when passengers were called with the cause of the extension, the response was generally positive and a good many were keen to be a part of such a flight, Captain Galvin explains.
The actual numbers for the 4,500 nautical mile sector between Perth and Johannesburg further reinforce the relative efficiency of carrying the spare engine. On this flight VH-OJS flew at the reduced Mach 0.78 and took 10 hours and 30 minutes to reach Johannesburg, burning 118,700kg of fuel in the process. By comparison, a flight without the fifth-pod would have taken 9 hours and 56 minutes and burned 109,500kg of fuel. That is a difference of around 10 tonnes of fuel and only 34 minutes.
The numbers for the fifth-pod stack up very well as an operational option to deliver a spare engine, equipment, tooling and engineering staff in a very timely manner. An engine could not have been air-freighted within the same time-frame and even then the leased engine would still have required the engineers to fly to South Africa for the installation. The carriage of a spare engine on a commercial service allowed the entire operation to remain in-house, cost-effective and with minimal disruption to the passengers.
The sight of an apparently ’five-engined’ Boeing 747-400 was enough to send a wave of interest across the internet. Pedestrians snapped iPhone shots as it passed overhead, while more dedicated aviation aficionados sought out vantage points and began online discussions. And while at first glance it may seem burdensome to attach an inert engine beneath a wing and drag it through the relative airflow, let’s not forget that the 747 has proudly flown with the Space Shuttle upon its back.
The fifth-pod operation is far from a common occurrence, but when the situation dictates, it is a very efficient means of returning a stranded airliner to the air. It can deliver the hardware, manpower and support equipment quickly, safely and in a commercially sound process. Operationally, it calls for co-operation across a spectrum of branches within the airline and the fact that this can occur in such a timely manner is testament to QANTAS’s long association with the procedure.
The carriage of an additional spare engine continues to fascinate the aviation world when it takes to the skies, apparently snubbing the laws of aerodynamics. However, the practice is deeply set in the histories of both Boeing and QANTAS and is founded on proven procedures and performance. And while it may be some time before QANTAS again needs to call upon the fifth-pod, it is good to know that there remains the ability to make a very special delivery.