The “Collective Cue” Danger and Rates of Descent
In the MH-60T helicopter, the “collective cue” on an instrument approach to the water commands the pilot to fly to the computer-generated glide path, regardless of the rate of descent necessary. If the helicopter is above glide path, the flight director cues will provide “commands” to descend back onto glidepath as rapid as necessary. I have seen “unstabilized approaches” where the pilot is lowering collective, the helicopter is descending at an excessive rate, and the collective cue is still a large down arrow.
In previous posts, we discussed that initiating the pitch attitude to start the deceleration, without initiating the appropriate power reduction, will result in a level speed change or a climb, both of which will put the helicopter above the computer-generated glide path. The MH-60T helicopter’s flight management system cues a deceleration at approximately 150’ AWL (specific altitude depends on glide path angle and groundspeed). If this transition is poorly executed (the helicopter levels off or climbs), the flight director provides a large down “collective cue” which, if followed, will result in high rates of descent at low altitudes.
Rates of descent – An instrument flying rule of thumb (technique) I rely on to maintain safety margins when in low or no visibility conditions is that rate of descent should not exceed your height above water (e.g., at 400’ AWL, rate of descent should not be greater than 400 fpm, 300’ never greater than 300 fpm, 200’ never greater than 200 fpm). Years ago, when our community had a procedure to hand fly an instrument approach to the water with no flight director cues, pilots were limited to no more than 400 fpm above 150’ AWL and no more than 200 fpm below 150’ AWL.
These values are still useful in recognizing an “unstabilized approach.” Our community now flies an approach that is between a 2- and 3-degree glide path (user selected). For 3-degree glide path, I have regularly harnessed the “rule of 5” – multiply your groundspeed by 5, which will give you the rate of descent necessary to stay on that glidepath. When the pilots start their approach at 300’ AWL at 70 kts groundspeed, that equals 350 fpm. After the helicopter’s deceleration around 150’ AWL when the groundspeed is below 40 kts, that equals 200 fpm. After that, the VSI should trend back to 100 fpm at 20 kts GS and fade out to 50 fpm around 10 kts GS.
An important point for new pilots in our community is that the slower the groundspeed, the easier it is to fly a stabilized instrument approach to a hover. Provided a consistent glide path, more wind equates to lower groundspeeds, which in turn creates milder rates of descent, resulting in a less dynamic, easier to stay ahead of, approach.
Furthermore, it is better to be slightly below glide path, rather than above, for two reasons:
1. Being slightly low keeps the rate of descent moderate, whereas being high can result in flight director “commands” for high rate of descent.
2. Being slightly low extends the horizontal lead distance programmed into the flight management system, whereas being high cuts into the horizontal obstacle clearance programmed into the approach.
