Engine Health Monitor (EHM) refers to a device that, installed in the blow by line of a piston aircraft engine, monitors the gas flow that by passes the piston rings (Blow By - BBY). It is a totally passive system, with no moving parts and whose functioning principle is based on the Pitot tube. As such, the BBY gas stream hits a cylindrical tube provided with orifices that sense the dynamic pressure exerted. Flush with the inside of the outer casing, other orifice(s) sense the static pressure. These two signals can be taken to the ports of a differential manometer or an aircraft air speed meter (ASI) of suitable range. It is possible in either case to calibrate the indication displayed in the most convenient form for the user. Several years of experience have been accumulated with the latter (ASI) on board of a small GA airplane. The original reading of the ASI in miles per hour (mph) was kept. As the internal diameter of the casing is known, the BBY indication in mph is easily converted in (liter/min) / cfm / inches of water or in any other suitable scale. The EHM in use was built with copper tubes drilled and soldered as an expedient way for a home-builder. However, with today's 3D printing capabilities, it can be built out of a heat resistant plastic with utmost ease, with advantages in reproducible output from unit-to-unit.

As those experienced on piston engine testing and technology know, BBY is the sole variable that has predictive capabilities. All others e.g exhaust temperature, merrily register what is already history, after the damage is already done. On the other hand, BBY trends can be monitored and subtle variations like a ring that is becoming stuck in its groove or that has an incipient crack that will progress to disaster if not interfered with, are reliably indicated by the EHM as an increase in BBY. The pilot can them act on the information given. Some subjective indications like vibrations or even the sound made by the engine are influenced by the air mass flown into. As an example, when crossing large bodies of water, the noise of a piston engine suddenly changes to a lower frequency. Understandably, this is a cause for anguish. With an EHM on board, just a glance reassures the pilot that all is well, the EHM showing a steady reading, the same as when previously flying over the country side. Conversely, lean mixtures, rings that are becoming worn, progressively failing exhaust or intake valves, a magneto out of proper timing etc, are all indicated by such simple device ahead of any damage. The EHM development was supervised by the FAA-Detroit and approved as a Form 337. Those from FAA involved were of the opinion that they (FAA) should make this device obligatory as a factory installed equipment. This possibility was pursued by the inventor.