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The ST-360 Altitude Selector/Alerter

by Jim Irwin, Vice President, Products

The S-TEC ST-360 Altitude Selector/Alerter is a great cockpit tool for reducing pilot workload. It therefore combines safety with utility and comfort, particularly for the pilot that flies a lot in IMC (Instrument Meteorological Conditions).

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The Selector/Alerter allows the pilot:
  1. to select a specific vertical speed for climb or descent,
  2. to preselect the next altitude, prior to takeoff or arriving at the TOD (top of descent), and to receive both aural and visual alerts as you near the selected altitude,
  3. to automatically accomplish altitude capture,
  4. to be alerted if the aircraft inadvertently leaves an assigned altitude,
  5. to select a DH (decision height) or an MDA (minimum descent altitude) and be alerted of nearing those altitudes,
  6. to maintain better airspace situational awareness,
  7. to monitor the outputs of the aircraft encoder.
Since the Selector/Alerter has quite a number of features and functions and a lot of buttons in a small package, it can be a little intimidating. However, the operation is very straightforward - let's have a look.

When powered up, the system comes up in "Baro" mode reminding the pilot to program the local altimeter setting into the selector. This allows reading the MSL (mean sea level) altitude in "ALT" mode and having the "ALT SEL" altitude referenced to MSL.

One valuable function is the ability to read your encoder output. This can be handy if you suspect your encoder of erratic or inaccurate outputs. To activate this, change the "Baro" calibration to 29.92 and select "ALT" mode. With the local altimeter setting in "Baro," the system corrects the encoder reference of 29.92 to the local setting, converting it to an MSL altitude, matching the altimeter.

In order to operate the system three basics need to be followed:

  1. Set the local barometer into the unit.
  2. Entering any value other than vertical speed requires "ENT" to be showing in the top left corner of the display.
  3. For normal operations, after making an entry, the "ENT" display must be extinguished for the system to work. This includes the vertical speed mode in which the Vs command to the autopilot will not operate in the "ENT" mode.
Altitude Select
After the local barometer setting is set on the display, push the "ALT" button to display the altitude select display ("ALT SEL"). To select the desired altitude, turn the rotary knob. Pulling the knob out allows the entry of altitudes in increments of hundreds of feet, i.e. 4.5 for forty five hundred feet.

Altitude
A second push of the "ALT" button will display encoder output adjusted for the current "Baro" setting. The display of the encoder output can be selected anytime without being in "ENT" mode by pushing the "ALT" mode button.

Alert
The "ALR" switch enables the altitude alert mode. Like the "ALT" mode, the "ALR" mode does not require a direct input so it can be activated without being in the "ENT" mode. The alert function operates in conjunction with a preselected altitude ("ALT SEL"). To get an alert 1000 ft prior and again 300 ft prior to reaching the selected altitude, push ALR and the "ALR" annunciation lights. To disable altitude alert, push the "ALR" button again and the "ALR" annunciation extinguishes. When alert altitudes are reached, a chime is heard through the cabin audio and the "ALR" annunciation flashes simultaneously.

Decision Height Alert Mode (DH)
The DH mode can be used for both precision (DH) and non-precision (MDA) IFR approaches as an altitude reminder. Since the use of the DH mode requires a data input, you must select the "ENT" mode with the "DTA" (data) button. With "DH" and "ENT" showing, the display will read "0.0." Turn the entry knob and choose an altitude to the nearest 100 ft which is immediately above the DH or MDA for the approach. i.e. If the DH/MDA is 1440 ft (MSL) you should choose 1500 ft or 1.5 (x1000). (Remember - encoders read altitude in 100 ft increments at the 50 ft points when they are operating properly.)

After the DH/MDA is input, press the "DTA" button again to extinguish the "ENT" annunciation. In about 5 seconds, the "DH" display will return to altitude (ALT) and will continue to display altitude as you descend. At 1550 ft you will receive an altitude alert as you enter the 100 ft above "DH" window. At 1450 ft you will receive a second alert indicating you are near or below the "DH."

Vertical Speed Mode
The "Vs" mode is normally the most frequently used. For "Vs" entries the "ENT" mode does not need to be activated. Simply select "Vs" (no "ENT" showing) to activate the display and rotate the knob to obtain the desired vertical speed with + for climbs and - for descents.

Important: This procedure sets the Altitude Selector/Alerter to output a "Vs" command, but the autopilot must have the "Vs" mode activated to respond to the output.

To operate the "Vs" mode select "Vs" on the autopilot programmer. Changes in vertical speed are commanded by rotating the knob to a higher or lower value. To disable the "Vs" function, press the "MAN" (manual) button on the Selector/Alerter. This extinguishes the "Vs" value selected and returns the flight of the aircraft to the autopilot "Vs" function. The "Vs" mode of the Selector/Alerter will return to manual anytime that the autopilot is turned off.

Altitude Select Operations
For automatic altitude select and capture, the pilot first sets the desired altitude and vertical speed on the Selector/Alerter and then simultaneously pushes the "Vs" and "ALT" buttons on the autopilot programmer. This selects "Vs" mode, with the Selector/Alerter commanding the climb or descent desired, while arming the "ALT" (altitude hold) mode for automatic engagement. When the altitude is captured the "Vs" mode annunciation in the autopilot will extinguish leaving only "ALT" altitude hold lit and engaged.

If you inadvertently input a "Vs" command of the wrong value for the altitude, the "Vs" Annunciator will flash as a caution. If you make an altitude change that is in the wrong direction for the vertical speed, the altitude will flash as a caution.

In the "ALT SEL" mode the Selector/Alerter is programmed to accomplish automatic leveling off at the selected altitude. This program reduces the vertical speed to 300 FPM as the aircraft nears the selected altitude. The initiation point for the level off program depends upon the rate of descent or climb. At high rates of climb/descent, the program could start as early as 900 ft prior to reaching the altitude, providing a very smooth leveling maneuver that will often go unnoticed by passengers.

Note: With the Selector/Alerter engaged, the rates of "Vs" selected may be limited by the level off program if you are relatively close to your selected altitude. i.e. If you are only 500 ft from your selected altitude, you cannot select a "Vs" higher than 700 FPM.

There you have it - a device to warn you as you approach an assigned altitude, or warn you if you inadvertently leave an assigned altitude, to alert you as you arrive at the decision height on a tight approach, that provides very smooth transitions to newly assigned altitudes and even provides a way to monitor encoder signals if you suspect a problem, and to help you maintain airspace situational awareness. The S-TEC Altitude Selector/Alerter is an attractive, all digital, highly reliable addition to your panel that reduces the workload of every flight. 

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