We report the instrument description and the results of the laboratory calibration and tests of a mid-infrared tunable diode spectrometer for in situ trace gas concentration measurements in the stratosphere operating on a stratospheric aircraft. The spectrometer is dedicated to the measurement of the HNO3 amount in the stratospheric aerosols by means of gas-phase absorption spectroscopy on molecular roto-vibrational lines in the mid-infrared, using a tunable diode laser and a multipass absorption cell. The instrument was specifically designed for operation aboard of the stratospheric aircraft M55 Geophysica, in the frame of the Airborne Platform for Earth observation (APE) project. The instrument is part of a measurement package for the measurement of the chemical content of Polar Stratospheric Clouds (PSCs) and other atmospheric aerosols. This system can be also used as a stand-alone detector of molecular trace gases. Design criteria include an efficient optical layout, with a very low sensitivity to the vibration and thermal stresses and a very small footprint, and a detection scheme based on the sweep integration technique for fast data acquisition and high signal-to-noise (S/N) ratio. We report a new set of testing measurements on ammonia as the calibration gas with one order of magnitude improvement with respect to what we previously reported.