So, I was watching a quick tutorial on the FMC on YouTube. The FMC somehow had a 5000 ft restriction, and the guy got clearance to go higher up, and he set his altitude on the AP panel, and he pressed the ALT INTV button.
From the FCOMv2 4.10.10 (emphasis mine):
Altitude Intervention (ALT INTV) Switch
[Option - With speed and altitude intervention]
Allows manual deletion of next FMC altitude constraint via altitude SEL and ALT INTV switch.
Push – (during VNAV climb)
- lowest FMC altitude constraint below selected MCP altitude is deleted
- if airplane is currently at an FMC altitude constraint, deletion allows airplane to resume climb. MCP altitude must be set above current altitude
- for each press of switch, one deletion occurs
- if MCP altitude is set above current FMC altitude, FMC cruise altitude resets to MCP altitude. FMC cruise altitude cannot be decreased using ALT INTV switch.
Push – (during VNAV cruise)
- if MCP altitude is set above current FMC cruise altitude, FMC resets cruise altitude to MCP altitude and initiates a cruise climb
- if MCP altitude is set below current FMC cruise altitude, an early descent is initiated. Lower FMC cruise altitude cannot be entered using ALT INTV switch.
Push – (during VNAV descent)
- the highest FMC altitude constraint above MCP altitude is deleted
- if airplane is currently at an FMC altitude constraint, deletion allows airplane to continue descent. MCP altitude must be set below current altitude
[Option - FMC U10.6 and above]
- if all FMC altitude constraints are deleted during VNAV path descent, an automatic transition to a VNAV speed descent is made.
For the 737NG it depends on the installed MCP (mode control panel).
- Honeywell (Sperry) has the INTV buttons
- Optional to remove them (bottom image)
- Collins (since 2003) does not have the INTV buttons (also see: b737.org.uk)
- Optional to add them (top image).
(Image source: 737NG FCOM)
When the ALT INTV button is not installed, the altitude selector knob can be pressed / pushed in. The same mechanism is also found on the 747-400, 757, 767, and 777. What the altitude intervention means is I want my selection now.
Let's say you're in cruise, and still some distance to go to the calculated top of descent (T/D) while flying in VNAV, then the ATC calls and instructs an early descent. Turning the knob to select a lower altitude is a signal to the VNAV to descend on its own when it reaches the T/D, but pressing the knob/button (depending on the model) commands that descent right away.
The same thing for climbs/descents when altitude restrictions are programmed for specific waypoints (e.g., SIDs and STARs).
Another variation used by Airbus and the MD-11/10 is the knobs having push and pull functions. For the altitude for example, a pull activates the FL CH mode equivalent, and a push is for the ALT INTV equivalent (Airbus) or altitude hold (MD-11/10).
It's quite simple to explain in laymen terms actually.
Before takeoff roll, the pilot input 5000 feet into the altitude window, then engaged VNAV1.
After takeoff, the plane will climb on the VNAV profile with an altitude restriction of "no more than 5000 feet", i.e. it will stop the climb and level at 5000 feet.
To climb higher, the pilot has to do two things:
- Rotate the altitude knob to set the altitude window to a higher value
- Push the "ALT INTV" button
If the pilot does not push the "ALT INTV" button, then rotating the knob has no effect at all. It is simply a command to tell the Auto Pilot to "execute" the new altitude restriction.
Here is more info about how the "ALT INTV" button is used.
You're cleared to climb to 5000 feet after takeoff (this instruction is given to you while you are still on the ground). You set the altitude window to 5000 feet, then takeoff. After a while, you are level at 5000 feet.
ATC clears you to 12000 feet. You rotate the knob to set 12000 feet, then push the button. The plane will now climb and level at 12000 feet.
Repeat until you reach cruise altitude.
On the way down, ATC clears to descend to FL 290. You rotate the knob to 29000 feet, then push the button. The plane will now descend from cruise altitude to FL 290.
Repeat until you are low enough to begin approach.
1: Technically VNAV can only be armed, but not engaged, while the plane is on the ground. It will automatically become engaged when the plane is 400 feet on the radio altimeter/AGL.