A very unusual aviation event took place on Wednesday January 5th 2016. A Qantas Boeing 747-400 (VH-OJH) normally powered with four Rolls Royce RB211-524 turbofans was seen flying with a 5th engine under its left wing.
The event was commented by a Qantas spokesperson via the web, indicating that due to another Qantas 747-400 being immobilised in Johannesburg, South Africa in urgent need of a replacement engine, a 5 Tonnes spare engine was simply bolted under wing of another 747-400 flight QF63 departing Sydney to Johannesburg for ferrying.
By reason of its tremendous size and weight, expedited shipment of a Rolls Royce RB211-524 turbofan can only be done in the main deck of a dedicated wide body cargo aircraft (or combi) plane. Except if there is an option to transport it externally fastened to the wing of a commercial 747 passenger plane. It seems Boeing engineers thought just that when they incorporated attachment points for a spare fifth engine on the venerable 747 way back in the 1960’s when designing the aircraft. The setup did not involve the need to power nor actually operate the engine but only have it travel securely bolted as an external under wing ‘passenger’.
The practice seems to have been undertaken on many occasions since the 1970’s first with the “Classic” series 747-100/-200/-300 and later with the 747-400. Sadly it was reported that Air India Boeing 747-200 flight 182 that exploded June 23rd 1985 due to a bomb being detonated on board killing all 329 inside was also carrying a spare engine on its left wing when the incident occurred. Pictures exist of various carriers operating 747s with the spare engine attached (notably a 2011 picture of Qantas own 747-400 VH-OJN sister ship of the VH-OJH involved in the latest ferrying flight).
The need to ferry a 5th engine mostly evolves from two situations: 1) a stranded 747-400 needs a spare engine. The spare is ferried to the stricken airliner strapped underneath the wing of another 747-400 or 2). a 747-400 that underwent emergency engine replacement (often leased or loaned from another carrier) while away from its base needs to ferry the unserviceable engine back to its home base where the faulty engine can be fully repaired.
3-engines ferry flights prohibit passenger flight
Yet 747 with have the option to conduct 3-engine ferry flight (back to their repair base) even when one engine is not functional. This may seem less expedient as it has to be done under very restrictive flight conditions involving specially qualified ferry flight crews operating under degraded safety limits margins in speed, weight and altitude. That option eliminates the possibility of generating revenue with paying passengers and goods transported on board. In all the ability to attach an engine under a regular commercial flight is just the least disruptive for flight operations, the most practical and economical by far as well as logistically expedient.
The Qantas announcement specified that the additional weight and drag incurred by the presence of an extra 5-Tonnes engine underneath the wing had warranted a refueling stop in Perth prior to the flight cruising directly to Johannesburg. The announcement made reference of the fact that the presence of that large, characteristically heavy outsize object under the aircraft’s left wing also necessitated adjustments being made by the flight crew of some of the aircraft flight configuration parameters.
Flight analysis and aircraft configuration (a 5th engine can induce as much as 7% less range for that specific mission)
According to flight data available in flightaware.com QF63 operated on January 6th 2015 undertook the Sydney Perth flight leg in 4 hours and 20 minutes and Perth-Johannesburg portion in 10 hours and 08 minutes. Normal QF63 direct flights Sydney-Johannesburg usually span a bit less than 13 and a half hours. The web site also reports an average speed of 445 knots (Mach 0.67) on the Perth leg and 463 knots (Mach 0.70, 857 kn/hr) when bound to Johannesburg. This compares to 495-499 knots (Mach 0.75, 920 km/hr) cruise speed normally on the route (these average speed incorporate slower take-off, climb, descent and approach phase so may be well below actual cruising speed under good or even adverse wind conditions). With a 5th engine, a 747-400 traveling at the optimized 857 km/hr speed on a 13.5 hour journey can only theoretically cover 11,569 km as opposed to the 12,420 km Sydney – Johannesburg span, representing a 7% range shortfall for that specific mission (actual distance flown directly between Sydney and Johannesburg is 6,857 nm; 11,035 km and additional reserve emergency fuel is required).
Does the spare engine needs to be covered with a aerodynamic nose cone?
-in cruise condition the flow of air going in and around the 5th engine can have serious drag penalty. it seems air rushing into the main fan blade can be a bit of an issue too.
For an idea of the thrust delivered by a 747 engine’s, pilots often rely on N1 value read out in their instrument panel. For Rolls Royce RB211-524G/H that power Qantas 747-400 fleet, a 100% N1 performance value on the pilot engine display indicates that the main fan rotation speed has reached 3,900rpm (rotation per minute). At maximum thrusts N1 values can reach 110.5%, equivalent to 4,310rpm. For the General Electric CF6-80C2-B1F turbofans equipping the 747-400ERs versions that Qantas also operates have N1 value of 100% that translate to 3,280rpm on the fan blade. Those engines have maximum fan blade rotation speed of 3,854 rpm reached when N1 is at 117.5%.
At high altitude cruise, the N1 value selected for thrust setting parameter depends on aircraft weight, altitude etc and only represents a fraction of the fan rotating speed mentioned above (N1 varying roughly anywhere in between 45% and 75%) however we can imagine that the windmill motion impelled by the aircraft speed onto the spare engine fan is meaningless (close to null), preventing high speed air from flowing freely through the fan blade back out through the fan cowling duct and the turbine, hence ultimately producing drag forces that are said to limit the aircraft performance to a speed number value between mach 0.78 and mach 0.81. We see that the 5th engine acting aerodynamically as somewhat of a brake chute under the left wing. Most agree that the proper way to allow a turbofan to travel economically and safely is to either remove the fan or cover it. Boeing wing maintenance manual illustrates this with a drawing of a cone-shaped engine cover fitted to the 5th engine. Even more, turbine blade may need to be secured to avoid windmilling altogether. This would prevent components damage due to frictions when no engine lubricant is supplied.
Here we must also recognize that the 747-100/-200 were designed to be very fast with purpose-built highly swept wing angled at 37.5 degree favoring an excellent cruise speed of mach 0.84. The stretched upper deck modification that first appeared on late model -200SUD (Stretched Upper Deck) in the mid 80s and standard on the later -300 and -400 series were said to provide aerodynamics benefits actually bumping cruise speed performance up a notch to mach 0.85.
Aircraft reconfiguration for flight with more drag on the left wing
The effect of drag induced by the presence of a 5th inoperative engine on the left wing can be modeled by the aircraft motion being slightly slower on the left wing, but faster on the right wing. Thus causing a flight attitude where the aircraft nose tends to turn slightly to the left.
The chrobotics.com web site explains that the difference between course and heading is called crab angle (very close to driving forward in a straight motion but with your head turned slightly to the side). For 747 this is big deal in high cross wind landing and also when operation with ‘one engine out’ requires assymetric thrust compensation.
It seems pilots can have options to compensate for a left turn tendency by trimming the aircraft to turn slightly right. Such trimming of the aircraft can be achieved by applying rudder deflection to the right. The net force resulting from more drag on the left wing and right side turn can cancel out each other and make the aircraft fly straight. This is done via rudder trim controls.
On 747-400 rudder trim ‘Yaw’ controls are located on the center aisle pedestal aft of the engine thrust levers. The crew can select the rudder deflection angle by turning a rotary knob to the desired position as indicated in the above Boeing diagram. This will result in the nose of the aircraft pointing precisely where the pilot intends it to.
The same mechanism also controls the deflection of the low and high speed aileron controlling bank angle trim. This can be applied by pilot input directly by rotating the steering wheel while activating a switch. However bank angle can not be applied past an angle value of 47 degree. Furthermore bank angle trim can not be applied while the auto-pilot is engaged. This where Yaw trim via rudder deflection can be very effective as it can be applied in auto pilot-managed cruising condition.
Aircraft reconfiguration for flight with more weight on the left wing
The addition of a 5th engine under the aircraft left wing does not pose structural issues for the aircraft however the extra 5 tonnes create obvious weight unbalances causing the aircraft to bank more to the left. This condition can easily be remedied by the introduction of a weight counter-balancing mechanism. On 747-400 the sophisticated fuel tank architecture permits to use embarked fuel as a flexible weight ballast system.
Fuel as a 5-Tonnes counter weight
The 747-400 fuel system architecture relies on a system of cross feed valves operated via the pilots overhead fuel management control panel. This links directly to a bus (intricate network of piping, valves and pumps to shift fuel from one tank to another)
the 747-400 fuel system architecture manages and distributes fuel among 8 tanks.
- 1 reserve in the horizontal stabilizer optional 9,992 kg
- 1 reserve in each of the wing tips optional 4,003 kg x 2
- 1 main huge tank in the centre wing tank 51,973 kg
- 1 main in each wing outboard section 13,572 kg x 2
- 1 main in each wing inboard section 37,989 kg x 2
- -400ER can have 2 additional fuel tanks installed in the aircraft belly cargo holds (carrying respectively 183,192 kg and 192,912 kg)
The diagram show that the 5th engine is located right under the aircraft left wing main fuel tank. With its 37,989 kg storing capacity, this fuel tank can be filled with just enough fuel to accommodate the extra weight of the 5th ferry engine. Simplistically put filling 5 tonnes less fuel on the left wing’s main tank than on the right wing’s main tank and the aircraft is balanced. But again we must keep in mind that the fuel management system can be a bit more elaborate as 747-400 have automated cross-feed features that continuously monitor fuel levels and can transfer it across different tanks.
Most material on the 747-400 systems in this article relies on the excellent Haynes Owners Workshop Manual BOEING 747 1970 onwards
by Chris Wood