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On Original, Classic, and Next Generation 737s, the APU panel has an EGT (exhaust-gas temperature) gauge below the four APU caution/warning lights (all images in this question are from the 737 Technical Site).1

Original / early Classic APU panel (Garrett 85-129 APU):

APU panel as it appears on an Original or an early Classic


Later Classic APU panel (Garrett 36-280 or Sundstrand APS-2000 APU; the NG uses the Allied Signal 131-9(B), whose panel lacks the ammeter gauge but is otherwise identical):

APU panel as it appears on later Classics


In contrast, the 737 MAX, although it uses almost exactly the same APU as the NG (still the [now Honeywell] 131-9(B), just with minor bugfixes), lacks the NG's EGT gauge:

APU panel as it appears on a MAX

Given that knowledge of EGT is important for flightcrew awareness of an APU overheat or other malfunction, why was the EGT gauge removed from the MAX's APU panel?


1: Also visible in these two photos is the APU ammeter, present only on Originals and Classics.

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  • $\begingroup$ @ymb1: I'm not seeing the APU panel anywhere in that picture you've linked to... $\endgroup$
    – Vikki
    Sep 26 at 0:10
  • $\begingroup$ @ymb1: I see two lights there (not four), and the switches for the APU and engine generators, but not for the APU itself. $\endgroup$
    – Vikki
    Sep 26 at 1:40
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    $\begingroup$ Really bad photoshop that last photo, see here for the missing knob. (Note: the lights are not illuminated.) $\endgroup$
    – ymb1
    Sep 26 at 1:46
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    $\begingroup$ On the 747 the APU EGT gauge was removed with the -400 variant (which removed the FE position). Since the 757, no analog APU EGT gauge is installed anymore. The EGT can be seen on the EICAS Status page on these aircraft, but that display is not available when starting the APU without external power. Airbus actually installed analog APU gauges on the A310 maintenance panel (back of the cockpit) for this reason, but they are removed since the A320. $\endgroup$
    – Bianfable
    Sep 26 at 9:12
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    $\begingroup$ Ahhh... gotcha. thanks for the clarification $\endgroup$
    – FreeMan
    Sep 29 at 11:39
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Given that knowledge of EGT is important for flightcrew awareness of an APU overheat or other malfunction.

All those are handled automatically by the APU's ECU (with built-in safeties). Same for the 737 NG series. So it was a gauge that can be done away with.

According to an FAA document from Nov 2020 on the 737-800 and 737-8 (aka Max) differences, which was "Approved by the Aircraft Evaluation Division", the removal of the APU EGT gauge does not impact the procedures or flight characteristics and is categorized as level-A ("Self-Instruction").

The FAA's approval – and lack of impacted procedures – should confirm my opening paragraph.

enter image description here

And here's a description from the NG's flight manual:

An Electronic Control Unit (ECU) monitors and controls the APU. Automatic shutdown protection is provided for overspeed conditions, low oil pressure, high oil temperature, APU fire, fuel control unit failure, EGT exceedance, and other system faults monitored by the ECU. The ECU automatically controls APU speed through the electronic fuel control. If speed or EGT exceed acceptable levels with the APU providing electrical load only, some electrical load is shed. When electrical load and air extraction raise the EGT above acceptable levels during engine starting, electrical load shedding occurs prior to reducing bleed air. When electrical load and air extraction raise the EGT above acceptable levels other than during engine starting, the inlet guide vanes move toward a closed position, reducing bleed air extraction while maintaining electrical load.

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You just never use that APU EGT gauge flying the NG. It’s almost pointless information as the APU ECU does it all. No non-normal checklists use it either.

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    $\begingroup$ Welcome to Aviation.SE! I removed your comment on MCAS since we try to keep answers on topic and free of opinion here. $\endgroup$
    – Bianfable
    Sep 27 at 8:06

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