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On the 737NG, will the cabin maintain a lower altitude than the plane or will cabin altitude equal aircraft altitude?
Will there be ANY cabin pressurization or none at all? When will the HORN sound?

I know this is extremely unlikely to happen but am posing the hypothetical question in order to better understand how the system works.

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For the curious, the first two sections explain the purpose of manually setting the FLT ALT.
You can skip directly to the answer section:

Intro

There are at least three scenarios to consider, not just one:

  1. Left on AUTO on ground with no selection (your question).
  2. Step climb (from cruise) without updating the selection.
  3. Descending before reaching the selection.

Now, let's see how the 737 differs from the typical newer jetliner (the 737NG's digital pressurization controller was optional on the 737 Classic):

  • In newer planes the pressurization controller is simply fed this data from the FMS (CDU or MCP entry).
  • Unlike other jetliners that maintain a cabin altitude with varying differential pressure (DP) for comfort, the 737 maintains a DP based on altitude blocks (consistent structural loads) when the FLT ALT is set. (If you don't have the manual, b737.org.uk covers that part and offers a good system description.)

Controller logic

Given that, the purpose of setting the FLT ALT on the 737 is maintaining consistent structural loads. The flight manual addresses all scenarios, but for a behind-the-scenes, I'll quote the controller logic from the patent (side note: 1981 matches the production start of the 737 Classic):

When the cruise altitude controller senses that the aircraft altitude change is within selected limits [when approx. 1,000 feet below FLT ALT], the system clamps to a scheduled value of cabin altitude [...]

If the aircraft cruise altitude drifts sufficiently to exceed ±500 feet from the altitude at which cruise control was imposed, the cabin altitude simply unclamps and the cabin climbs or descends at a predetermined rate under the control of central processing unit 54. [...]

Central processing unit 54 computes the differences between the pressure altitude and the value of Hcruise-clamp once per minute. If that difference exceeds the predetermined value, Hcruise-clamp is assigned the value of the pressure altitude. [...]

Direct answer

cabin altitude vs flt alt vs altitude
Source: 737 flight manual

As you see, the FLT ALT value only matters as the plane approaches it (which the controller checks for periodically), else (above/below) the schedule is automatically maintained, so:

The controller will still pressurize the cabin – but not in reference to the DP schedule when cruise is reached, rather in reference to the current pressure altitude.

For an early descent, the controller will engage the off-schedule descent mode – landing altitude being the same as the departure airport altitude that was automatically sensed and set in ground mode – and the crew will be alerted to that. (This is helpful for a diversion/return before reaching the cruise altitude.)

enter image description here
Off-schedule descent warning, modified from b737.org.uk

Also, as the illustration shows, in a descent before reaching ~1000 ft below the set FLT ALT, the controller will still reduce the cabin altitude (but not in descent mode).

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