Shubnikov de Haas resistance oscillations of highly mobile two dimensional helical electrons propagating on a conducting surface of strained HgTe 3D topological insulator are studied in magnetic fields B tilted by angle θ from the normal to the conducting layer. Strong decrease of oscillation amplitude A is observed with the tilt: A∼exp(−ξ/cos(θ)), where ξ is a constant. Evolution of the oscillations with temperature T shows that the parameter ξ contains two terms: ξ= ξ1+ ξ2T. The temperature independent term, ξ1, signals possible reduction of electron mean free path lq and/or enhancement of in-homogeneous broadening of the oscillations in magnetic field B. The temperature dependent term, ξ2T, indicates increase of the reciprocal velocity of 2D helical electrons: δ(vF−1)∼B suggesting modification of the electron spectrum in magnetic fields. Results are found in good agreement with proposed phenomenological model.