Monitoring an Auto Approach (PATCH)
What is the difference in the MH-60T autopilot functionality when you only have SAS1 compared to when you only have SAS2?
At 0200 on the dark and stormy night, the easy to remember answer is that when SAS2 is lost, nothing engaged on the second row of buttons on the AFCS panel will be working, which includes altitude holds (BARALT and RADALT), auto approach and auto depart (*** asterisks on this one*** some auto depart functionality remains, but if a SAS2 malfunction exists, I recommend a manual ITO).
An old flight examiner trick is to reach down and turn off SAS2 during a PATCH (auto approach) to see when the pilot detects anomalies and if they take appropriate action (i.e., abort the approach and go around using a manual ITO, avoiding terrain and obstacles). As a result of this trick, many pilots (including me) develop methods to detect that SAS2 has been secured and what I have realized while flying operational PATCHs without any visual cues, is that the methods employed to subvert flight examiner tricks have the intended effect of instilling confidence that the autopilot is functioning well.
To discern whether a pilot can recognize an abnormal PATCH, I like to ask pilots how they know the PATCH is not working. One answer I receive is “the helicopter does not decelerate by XXX feet” – this is true, but XXX feet is variable depending on three factors – the groundspeed/wind present, the altitude selected on the potentiometer, and the longitudinal groundspeed selected on the potentiometer.
The lower your groundspeed/higher the wind, the lower the transition will occur. The higher the selected hover altitude in the potentiometer, the higher the transition will occur. The higher the selected groundspeed in the potentiometer, the lower the transition will occur. So, while it is difficult to pinpoint an altitude, the helicopter should “flare” between 100-200’ AWL.
Another answer pilots give is that the helicopter’s “100 foot” annunciation (CAWS notification), which is silenced or suppressed during an auto approach, is heard. If the crew hears the CAWS “100 feet” annunciation during a PATCH, the auto approach is not working appropriately, and the helicopter will not stop the descent without pilot intervention. Even though this is a great stop gap clue, ideally, an approach anomaly is recognized before 100’ AWL as this is a low altitude to determine to abort an approach, but better late than never.
While it is important for pilots to recognize that both indicators signify an abnormal auto approach, a better way to monitor the PATCH is by tracking the descent rate. The MH-60T flight manual addresses rates of deceleration and rates of descent, however, rates of deceleration (2.5 KT/SEC above 40 KT GS, 1.5 KT GS below 40 KT GS), while good for situational awareness, are only slightly useful as they cannot be monitored with flight instruments. Instead, pilots familiarize themselves with the helicopter’s response. The deceleration is initially more aggressive and then fades out as inertia is bled off in the “flare” and the helicopter slows. Pilots feel this in the helicopter. On no wind days (e.g., 70 KIAS = 70 KT GS) the initial deceleration is more aggressive for a longer period when compared to high wind days (e.g., 70 KIAS = 45 KT GS). That said, without an effective method to track rates of deceleration and determining such rates during a PATCH is unrealistic, pilots should defer to the descent rate.
Rates of descent are stipulated in the flight manual (360 FPM “above line approach,” 215 FPM above 40 KT GS, 130 FPM below 40 KT GS) and the helicopter is equipped with a rate of descent instrument (the VSI), which pilots should be monitoring during a PATCH. Therefore, knowing the rates of descent is critical to monitoring the approach and, from my experience, a great way to ensure the helicopter is functioning properly, not only on that check ride when the flight examiner turns SAS2 off, but MORE IMPORTANTLY, on that dark foggy night as the helicopter approaches the water with no visual references.
Although the flight manual references specific numbers with respect to rates of descent on an auto approach, operationally these are considered general guidelines. In practice, when a pilot presses the auto approach button, the aircraft initiates approximately a 400 FPM rate of descent (this is the FM 360 FPM rate of descent “above line approach”), which then fades out and transitions to a stable rate of descent between 200-300 FPM (215 FPM above 40 KT GS). At some point between 100-200’, depending on the variables mentioned above (roughly around 150’), the pitch attitude increases, the power decreases, the VSI shows a more abrupt descent for a second, then as power comes in, rate of descent stabilizes between 100-200 FPM. At approximately 15 feet above the altitude set in the potentiometer, the rate of descent should fade out (this is from the FCF procedures *** potential flight manual change*** this is useful information for all operators).
Now that these standard rates of descent have been highlighted so that pilots are better equipped at monitoring the approach, next I will address some techniques for PATCH implementation.
