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Recently, some crew members inquired about the issue of the automatic throttle pre-position light not illuminating on the MCP panel. In fact, this is a normal phenomenon for aircraft equipped with HONEYWELL MCP. We have shared this before.
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Incident Background
On February 26, 2017, a 737NG aircraft had a short stop in Shijiazhuang. After the aircraft door was closed, the crew reported that the A/T integration ARM light did not illuminate after engaging A/T.
The light test also did not illuminate the ARM light, but the pre-position switch did not disengage. They thought the A/T ARM light was faulty, but later realized this phenomenon is normal and not a fault. The aircraft departed normally.
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Two Types of MCP Panels
On the MCP panel, above the A/T ARM switch, there is an automatic throttle pre-position light which serves as a prompt for the crew when the automatic throttle is pre-positioned.
Under normal circumstances, when the A/T ARM switch is set to the ARM position, the green A/T pre-position light illuminates, indicating that A/T is pre-positioned.
If the pre-position conditions are met, the electromagnetic coil will keep the switch in the pre-positioned state, and the A/T pre-position light will remain illuminated.
If the FCC A (or the A/T computer on older aircraft) detects a problem with the automatic throttle pre-position, the switch will automatically return to the OFF position, and the A/T pre-position light will go out.
Currently, the company’s 737NG fleet is equipped with MCP panels from two manufacturers: HONEYWELL (4082260-937/939) and COLLINS (822-1567-101/102).
The design philosophy of the A/T ARM light differs between HONEYWELL and COLLINS MCP panels, leading to misunderstandings among crew or maintenance personnel regarding the illumination of this light.
Refer to 737-SL-22-052:
INTERCHANGEABILITY
The Rockwell Collins MCP and Honeywell MCP are two-way interchangeable on all 737NG airplanes with the following exceptions:
The Honeywell parts are not interchangeable with the Rockwell Collins part if the MDA Altitude Select Option is activated. (If the Honeywell part is inadvertently installed on an airplane with the MDA Altitude Select Option activated, the MDA altitude will not appear as an automatic detent during altitude selection.)
If the CATIIIb option is activated and the Honeywell parts are installed, the highest autoland capability will be LAND 2 as indicated by the NO LAND 3 annunciation. (Status downgrade will be automatic.)
The A/T ARM switch light annunciation is different between the Rockwell Collins and Honeywell MCPs. The Honeywell annunciation followed the 737-300/400/500 MCP design and the light is intended to indicate the switch position in low light/darkness. While the design requirements did not change, the switch light annunciation in the Rockwell Collins MCP was implemented to be identical to the other MCP mode annunciations. Therefore, the A/T ARM switch annunciation on the Rockwell Collins MCP will appear brighter than the Honeywell MCP in daylight/sunlight conditions. In addition, the A/T ARM switch light will illuminate on the Rockwell Collins MCP when the Master Test and Dimming Switch is set to the TEST position. The switch light on the Honeywell MCP will not annunciate for this condition.
For HONEYWELL MCP panels:
The A/T ARM light is designed to have a lower operational brightness. Its design concept is to prompt the crew whether the A/T ARM switch is in the ARM position in low light or darkness.
A/T ARM light illumination conditions:
① The main instrument panel light control knob on P1-5 panel is set to the bright position
② The automatic throttle is set to pre-position
③ The light test does not illuminate it
↑ P1-5 panel MAIN PANEL BRIGHT set to maximum
In normal daylight, the brightness of this light is very low and almost invisible (unless the brightness knob is set to full brightness; due to the relatively low brightness, one may need to get closer and shield the sunlight to observe it). The crew can directly check the actual position of the A/T ARM switch to confirm whether the automatic throttle is pre-positioned.
For COLLINS MCP panels:
On COLLINS MCP panels, when the automatic throttle is pre-positioned, the automatic throttle pre-position light will illuminate, and the light test will also illuminate it.
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Maintenance Tips
The MCP panels manufactured by HONEYWELL and COLLINS can be interchangeable under certain conditions. The current manual is based on the MCP configuration when the aircraft was delivered and does not effectively control the two types of MCP panels, thusthe illumination of the automatic throttle pre-position light depends on the differences in MCP panel configuration, and cannot be judged by the aircraft number.
There are also very obvious differences in the appearance of MCP panels from the two manufacturers.
↑ Figure 1: HONEYWELL MCP Panel
① A/T pre-position switch is a flat lever
② Mode selection button is a flat rectangular shape
③ Button identification text is outside the button
↑ Figure 2: COLLINS MCP Panel
① A/T pre-position switch is a cylindrical lever
② Mode selection button is a vertical rectangular shape
③ Button identification text is on the button
▐ Differences in Automatic Throttle Pre-Position Light
When the automatic throttle pre-position light does not illuminate, first clarify whether it is a HONEYWELL or a COLLINS MCP panel.
If it is confirmed to be a HONEYWELL MCP, then further adjust the brightness control knob on the P1-5 panel to the maximum brightness, and place the A/T pre-position switch in the ARM position to see if the light can illuminate.
Note: During the day, one may need to get closer and shield the sunlight to observe.
If the pre-position light can illuminate, it indicates normal operation.
If the pre-position light cannot illuminate, and the automatic throttle disengagement light (red) does not illuminate, and the automatic throttle pre-position switch does not automatically return to the OFF position, then the automatic throttle integration is normal. The automatic throttle pre-position light can be retained according to MEL22-18-03.
For COLLINS MCP panels, if the A/T ARM switch is placed in the ARM position and the pre-position light does not illuminate, and the automatic throttle disengagement light (red) does not illuminate, and the automatic throttle pre-position switch does not automatically return to the OFF position, then the automatic throttle integration is normal. The automatic throttle pre-position light needs to be retained according to MEL22-18-03.
▐ Other Configuration Differences of MCP Panel
Speed Intervention and Altitude Intervention Switches
SPD INTV and ALT INTV
The appearance of four types of part number panels is as follows:
↑ COLLINS, PN: 822-1567-102/-104
↑ COLLINS, PN: 822-1567-101/-103 (Speed and altitude intervention switches are blocked)
↑ HONEYWELL, PN: 4082260-939
↑ HONEYWELL, PN: 4082260-937(Speed and altitude intervention switches are blocked)
Note that panels with speed intervention and altitude intervention functions cannot be interchanged with those without these functions, meaning they must be consistent with the aircraft’s speed intervention and altitude intervention function options.
End
Recommended Reading:
Maintenance Tips | “NO VSPD” Message on 737NG Aircraft
Interchangeability of Four Part Numbers for Inertial Computers on 737NG Aircraft
737NG Inertial Mode Selection Panel, Do You Really Know How to Use It?
