  This study concerns a turbulent jet at Mach number $M_j=0.9$, subject to a
uniform external flow stream at $M_f=0.15$. We assess the mechanisms that
underpin the reduction in fluctuation energy that is known to occur when a jet
is surrounded by a flight stream. The analysis combines experimental and
numerical databases, spectral proper orthogonal decomposition (SPOD) and linear
modelling. The experiments involve Time-Resolved, Stereo PIV measurements at
different cross-sections of the jet. A companion large-eddy simulation was
performed with the same operating conditions using the "CharLES" solver by
Cascade Technologies in order to obtain a complete and highly resolved 3D
database. We show that the energy reduction is spread over a broad region of
the frequency-wavenumber space and involves, apart from the known stabilization
of the modal Kelvin-Helmholtz (KH) instability, the attenuation of flow
structures associated with the non-modal Orr and lift-up mechanisms. Streaky
structures, associated with helical azimuthal wavenumbers and very slow time
scales, are the most strongly affected by the flight stream, in terms of energy
attenuation and spatial distortion. The energy reductions are accompanied by a
weakening of the low-rank behaviour of the jet dynamics revealed by previous
studies. These trends are found to be consistent with results of a local linear
model based on the modified mean flow in the flight stream case.
