The modern Non-Intrusive Inspection Systems (NIIS) are the backbone of air transportation security. The commercial aviation is impossible without them. Real-time evaluation of the NIIS technical condition is critical for the airport operators. Most of the operated worldwide NIIS were designed before the Internet of Things (IoT) era, and they don’t have the options to provide real-time monitoring of their operational status. The examined X-ray generator, one of the most common types, is equipped with a stationary anode X-ray tube, mounted on an HS 100100V (Smiths Heimann Gmbh). The X-ray tube is immersed in cooling oil bath and encapsulated in stainless steel hull. The set of installed sensors consists of contact sensor arrays combined with a distant infrared sensors system for mapping the outer surface X-ray generator temperature profile. Real-live operational conditions were simulated to study steady-state thermal characteristics. Two indicators for normal operation with sufficient conservatism were selected for suitable monitoring of the operability of X-ray generator:the maximal allowed oil volume expansion and maximal long-term temperature at the inner oil volume boundary. They were estimated by the reported in this paper analytical 1-D thermal model after measurement of the outer hull's surface temperature. As a results, we distinguished four modes of X-ray generator operation: (1): 'normal operation;' (2): 'slow degradation;' (3): 'faster degradation;' and (4): 'failure.' The maximal temperatures on the hull's outer surface were estimated for each mode as follows: (1): 10...51°C, (2): 51...80°C, (3): 80...87°C. The failure can take place at temperatures above 87°C. We found that the 'normal operation' mode could be extended up to 60°C due to oil thermal expansion.