In the current trend toward thicker aluminium plates, a major concern is the generation of high internal stresses during quenching, which can cause distortions during machining and pose serious safety concerns. Although the material is stretched after quench, substantially reducing residual stresses, they are not fully suppressed. In addition, the cooling rate is not large enough at the core of such thick plates to prevent any precipitation. This has a great impact on the efficiency of ageing. In this work, residual stress distributions in a heat-treatable aluminium alloy AA7449 thick plate in the as-quenched state measured by neutron diffraction are presented. A comparison between single (311) diffraction peak and multiple peaks analysis using Pawley algorithm is shown. The variation of the stress free reference value through the plate thickness is discussed and measured stresses are compared with residual stresses predicted by a thermomechanical finite element model of quenching.