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<span id="openzim-page-title" class="mw-page-title-main"><span class="mw-page-title-main">Tach timer</span></span>
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<p>The <b>tach(ometer) timer</b> is an instrument used in <a href="Aviation" title="Aviation">aviation</a> to accumulate the total number of <a href="Revolutions" class="mw-redirect" title="Revolutions">revolutions</a> performed by the engine. The unit of measure is equivalent to the number of hours of running at a certain, specific reference speed of rotation. If the reference speed of rotation is 2400 <a href="Revolutions_per_minute" title="Revolutions per minute">RPM</a> then the timer runs in <a href="Real-time_computing" title="Real-time computing">real time</a> when the engine is running at 2400 RPM, half speed while the engine is run at 1200 RPM (a fast idle for some aviation engines) or at 5/6ths real time at 2000 RPM (a slow cruise speed).<sup id="cite_ref-1" class="reference"><a href="#cite_note-1"><span class="cite-bracket">[</span>1<span class="cite-bracket">]</span></a></sup>
The tach timer integrates over time the instantaneous rotation speed displayed by the <a href="Tachometer" title="Tachometer">tachometer</a>. The displayed number is incremented by one if the engine is run at its reference speed for one hour. The quantity recorded is referred to as tach(ometer) hours. If the reference rotation speed is 2400 RPM then the tach timer records
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<dl><dd><span class="mwe-math-element mwe-math-element-inline"><span class="mwe-math-mathml-inline mwe-math-mathml-a11y" style="display: none;"><math xmlns="http://www.w3.org/1998/Math/MathML" alttext="{\displaystyle {\frac {{\text{total}}\ {\text{revolutions}}}{2400\cdot 60}}}">
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<mtext>total</mtext>
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<mtext>revolutions</mtext>
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<mn>2400</mn>
<mo>⋅<!-- ⋅ --></mo>
<mn>60</mn>
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<annotation encoding="application/x-tex">{\displaystyle {\frac {{\text{total}}\ {\text{revolutions}}}{2400\cdot 60}}}</annotation>
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</math></span><img src="./bde937f08202ecda49e33d938a032bcf00a8881a.svg" class="mwe-math-fallback-image-inline mw-invert skin-invert" aria-hidden="true" style="vertical-align: -2.005ex; width:17.394ex; height:5.509ex;" alt="{\displaystyle {\frac {{\text{total}}\ {\text{revolutions}}}{2400\cdot 60}}}" loading="lazy"></span></dd></dl>
<div class="mw-heading mw-heading2"><h2 id="Uses">Uses</h2></div>
<p>The tach timer is usually used to schedule engine maintenance, although it is just an approximation of "Time in service" which is used to time and schedule aircraft maintenance. Time in service is defined in 14 CFR 1.1<sup id="cite_ref-2" class="reference"><a href="#cite_note-2"><span class="cite-bracket">[</span>2<span class="cite-bracket">]</span></a></sup> as the actual time in the air, whereas tach time measures engine revolutions, which would still count time on the ground while the engine is idling (at a lower rate).<br>
It can also be used as a basis for charging for aircraft rental as opposed to charging for elapsed time. This encourages the renter to properly warm the engine before takeoff and not to run the engine continuously at maximum speed.
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<div class="mw-heading mw-heading2"><h2 id="See_also">See also</h2></div>
<ul><li><a href="Hobbs_meter" title="Hobbs meter">Hobbs meter</a></li></ul>
<div class="mw-heading mw-heading2"><h2 id="References">References</h2></div>
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<li id="cite_note-1"><span class="mw-cite-backlink"><b><a href="#cite_ref-1">^</a></b></span> <span class="reference-text"><style data-mw-deduplicate="TemplateStyles:r1238218222">
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</style><cite class="citation web cs1"><a rel="nofollow" class="external text" href="http://foxflying.com/FAQ.htm#WhatIsTach">"Frequently Asked Questions"</a>. Fox Flying Club<span class="reference-accessdate">. Retrieved <span class="nowrap">1 December</span> 2011</span>.</cite></span>
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<li id="cite_note-2"><span class="mw-cite-backlink"><b><a href="#cite_ref-2">^</a></b></span> <span class="reference-text"><cite class="citation web cs1"><a rel="nofollow" class="external text" href="https://www.ecfr.gov/cgi-bin/text-idx?rgn=div8&node=14:1.0.1.1.1.0.1.1">"14 CFR 1.1"</a>. U.S. Government Publishing Office<span class="reference-accessdate">. Retrieved <span class="nowrap">9 August</span> 2017</span>.</cite></span>
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