Help me understand CHT requirements

[rpadula]

Last night I was looking at engine monitors (JPI, EI, Insight), and noticed that some of them made special mention of being FAA approved for Primary temperature instrument replacements.

What is special about the CHT gauge that would make them say this? It's not currently listed as a required instrument in 91.205.

I also noticed that some -- for example, the JPI EDM 711 -- were STC'd for my aircraft (Commander 112) and some were not (oddly, the EDM 700/800 by the same manufacturer). Does an instrument have to be STC'd for the aircraft in order to be used as a primary?

And, once a primary replacement instrument is installed, can you remove the original factory gauge?

Thanks.

 

[Areeda]

We just got a JPI EDM 930 in our club Arrow. All the analog gauges were removed.
It is pretty cool.

I don't know the answer to your question about the STC.

Joe

 

[MikePapa]

The JPI does a lot more than just measure CHT.

§ 91.205

(b) Visual-flight rules (day). For VFR flight during the day, the following instruments and equipment are required:
...
(7) Oil temperature gauge for each air-cooled engine.

 

[wsuffa]

Last night I was looking at engine monitors (JPI, EI, Insight), and noticed that some of them made special mention of being FAA approved for Primary temperature instrument replacements.

What is special about the CHT gauge that would make them say this? It's not currently listed as a required instrument in 91.205.

I also noticed that some -- for example, the JPI EDM 711 -- were STC'd for my aircraft (Commander 112) and some were not (oddly, the EDM 700/800 by the same manufacturer). Does an instrument have to be STC'd for the aircraft in order to be used as a primary?

And, once a primary replacement instrument is installed, can you remove the original factory gauge?

Thanks.

For some aircraft CHT is required. IIRC, the Commander has a CHT gauge requirement.

STC is generally required - if it's not STCd for your aircraft, you gotta go through field approval.

You can remove the factory primary once you install a primary replacement. On my plane, the TIT instrument and the voltmeter/ammeter instrument are EI primaries and the factory stuff has been removed. I've been thinking about the EI MVP-50, and pulling the JPI and all the other primary guages. That should cut some weight. My concern is the reliability (and being stuck somewhere) given that I've had some recurring probe issues with the JPI.

 

[sba55]

In many HP planes, CHT instruments are installed by the factory and then you can usually only remove them if you install a certified replacement. It's a good idea anyways because the factory instruments are usually wildly inaccurate and read really low.

 

[CJones]

Or just build a homebuilt and you can use a meat thermometer from Wal-Mart for your CHT gauge.

 

[MikePapa]

Or just build a homebuilt and you can use a meat thermometer from Wal-Mart for your CHT gauge.

That's really all a CHT guage is anyway, even in certified aircraft.

 

[poadeleted20]

Since 91.205 doesn't require a CHT in your plane, here's the deal:

First, is there a CHT gauge in your plane already? If not, then there's nothing to worry about -- you can add the JPI instrument without worrying about it being a "primary instrument replacement" -- no STC or field approval required. If there is, then you have to check your aircraft's equipment list to see if it's required or just standard/optional. If standard/optional, then there's nothing to worry about -- you can relpace it with the JPI instrument without worrying about it being a "primary instrument replacement."

OTOH, if you have a CHT gauge and it's listed as required in the aircraft paperwork, then you have three choices:
1. Leave the old CHT in and add the JPI.
2. Replace the old CHT with an STC'd item and write it up on a 337.
3. Replace the old CHT with a non-STC'd replacement and get it field approved on a 337.

 

[RotorAndWing]

Since 91.205 doesn't require a CHT in your plane, here's the deal:

First, is there a CHT gauge in your plane already? If not, then there's nothing to worry about -- you can add the JPI instrument without worrying about it being a "primary instrument replacement" -- no STC or field approval required.

What is required is a source of approved data to make an installation and a log book entry.

You can use AC 42.13-2B Chapter 11 if the aircraft maintenance manual does not have the data within. For CAR3 airplanes you would use:

CAR 3.675 Cylinder head temperature indicating system for air-cooled engines. A cylinder head temperature indicator shall be provided for each engine on airplanes equipped with cowl flaps. In the case of airplanes which do not have cowl flaps, an indicator shall be provided if compliance with the provisions of § 3.581 is demonstrated at a speed in excess of the speed of best rate of climb.


For a FAR 23 airplane you need to use FAR 23.1321:

23.1321 Arrangement and visibility.

(a) Each flight, navigation, and powerplant instrument for use by any required pilot during takeoff, initial climb, final approach, and landing must be located so that any pilot seated at the controls can monitor the airplane's flight path and these instruments with minimum head and eye movement. The powerplant instruments for these flight conditions are those needed to set power within powerplant limitations.
(b) For each multiengine airplane, identical powerplant instruments must be located so as to prevent confusion as to which engine each instrument relates.
(c) Instrument panel vibration may not damage, or impair the accuracy of, any instrument.
(d) For each airplane, the flight instruments required by §23.1303, and, as applicable, by the operating rules of this chapter, must be grouped on the instrument panel and centered as nearly as practicable about the vertical plane of each required pilot's forward vision. In addition:
(1) The instrument that most effectively indicates the attitude must be on the panel in the top center position;
(2) The instrument that most effectively indicates airspeed must be adjacent to and directly to the left of the instrument in the top center position;
(3) The instrument that most effectively indicates altitude must be adjacent to and directly to the right of the instrument in the top center position;
(4) The instrument that most effectively indicates direction of flight, other than the magnetic direction indicator required by §23.1303(c), must be adjacent to and directly below the instrument in the top center position; and
(5) Electronic display indicators may be used for compliance with paragraphs (d)(1) through (d)(4) of this section when such displays comply with requirements in §23.1311.
(e) If a visual indicator is provided to indicate malfunction of an instrument, it must be effective under all probable cockpit lighting conditions.


and FAR 23.1305


23.1305 Powerplant instruments.

The following are required powerplant instruments:
(a) For all airplanes. (1) A fuel quantity indicator for each fuel tank, installed in accordance with §23.1337(b).
(2) An oil pressure indicator for each engine.
(3) An oil temperature indicator for each engine.
(4) An oil quantity measuring device for each oil tank which meets the requirements of §23.1337(d).
(5) A fire warning means for those airplanes required to comply with §23.1203.
(b) For reciprocating engine-powered airplanes. In addition to the powerplant instruments required by paragraph (a) of this section, the following powerplant instruments are required:
(1) An induction system air temperature indicator for each engine equipped with a preheater and having induction air temperature limitations that can be exceeded with preheat.
(2) A tachometer indicator for each engine.
(3) A cylinder head temperature indicator for—
(i) Each air-cooled engine with cowl flaps;