Only the RB211-524G powered 747-400s (ie, the -438s) are certified to carry a spare engine (or 'fifth pod'). The engine is fitted inboard of Eng 2.
The presence of a heavy additional load on one wing only entails several operational restrictions and considerations. Firstly, and most obviously, is the additional drag and weight (6524kg for a -524G-T engine) acting asymmetrically on the aeroplane. This requires considerable rudder and aileron trim to neutralise. We found that presetting one unit of right rudder trim (as required by the performance manual) gave the aeroplane an almost identical feel to a clean aeroplane during the departure and approach phases.
Secondly, the presence of such a sizeable object on the underside of the wing root (the most significant lift-generating section of the wing) imposed additional aerodynamic penalties - stall speeds, buffet boundaries and indeed the entire flight envelope are modified (in a restrictive sense), and (obviously) fuel flows were substantially greater. The aircraft's Flight Management Computer has a 'Spare Engine' mode, in which the relevant performance data and aerodynamic limitations appropriate to operations with a fifth pod are used in lieu of conventional 'clean' performance data. The activation of this function is annunciated both on the FMC CDU Ident page ('Spare Engine Active') and as an upper eicas memo ('VMO SPARE ENGINE').
Maximum airspeeds are reduced for fifth pod operations - the performance manual recommends a maximum 330/M0.78 (although the stated absolute MMO is 0.85). Speeds greater than M0.78 are likely to generate significant airframe buffet - indeed we felt moderate buffet throughout the cruise as the airstream broke down around the engine pod.
The front fan was removed (to reduce drag), and an 'ice deflector' fitted to avoid accumulation of ice inside the 'hot' area of the engine during flight through icing conditions.
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