  The Transit Timing Variation (TTV) technique is a powerful dynamical tool to
measure exoplanetary masses by analysing transit light curves. We assessed the
transit timing performances of the Ariel Fine Guidance Sensors (FGS1/2) based
on the simulated light curve of a bright, 55 Cnc, and faint, K2-24,
planet-hosting star. We estimated through a Markov-Chain Monte-Carlo analysis
the transit time uncertainty at the nominal cadence of 1 second and, as a
comparison, at a 30 and 60-s cadence. We found that at the nominal cadence
Ariel will be able to measure the transit time with a precision of about 12s
and 34s, for a star as bright as 55 Cnc and K2-24, respectively. We then ran
dynamical simulations, also including the Ariel timing errors, and we found an
improvement on the measurement of planetary masses of about $20-30\%$ in a
K2-24-like planetary system through TTVs. We also simulated the conditions that
allow us to detect the TTV signal induced by an hypothetical external perturber
within the mass range between Earth and Neptune using 10 transit light curves
by Ariel.
