Expert: Ismar Avdic, Eng. - 10/13/2006

Question
Hi Ismar; I read your answer on the requirement of Hobbs meters with interest. My question is: To what is the Hobbs measurement linked to give engine time - if it is linked to the Tacho., how are hours calculated? Is it on cruise revs, maximum revs, or is it a chronological measurement?

Thanks,

Peter  

Answer
Peter
You see Hobbs meter is seperate unit that measure the time the system is activated. I have never disassembe an unit, it would make him unairworthy but there is facts about Hobbs meter that I can relate. Engine time is measured from different unit Tachometer, so two of them are not connected. Hobbs meter is separate unit and it is triggered with oil pressure that unit senses and starts to count hours (new versions of Hobbs meters). Old version Hobbs meter was just hour meter that was initiated by MASTER switch. Lot of unhonest pilots what they did when they take off in uncontrolled airports tehy would shut down MASTER switch so they can save money or cheat owner. However new version has small adjustment that has to be hook up with engine oil pressure line and senses engine oil pressure when engine starts till shut down and measures hours.
It makes very easy to stop that clicking sound from Hobbs meter-just shut down engine. Well I bet a lo of poeple will think twice before they do that after take off. LOL
Hoobs meter measures tenth of hour. or .1 is 6 minutes from his internal time measuring system. So in reality you will never have both Hobbs and Tachometer same reading, unless you have some airplane from Twilight Zone, LOL.

Regards
Ismar