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Normal/Steep Approach: Rate of Descent


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I'd like some CFIs to weigh in on the topic of normal and steep approaches, and their associated rate of descent. The way I teach my normal approaches is to maintain a descent angle using a sight picture. (Compass level on the 300C) Meanwhile, maintain an apparent brisk walk rate of closer. (Fast at 300', slower as you descend.)

 

Now, headwind component plays a big part in determining what the VSI will indicate as we fly this approach. In a NO WIND condition, the VSI may read upwards of 700 fpm. This makes some nervous, as they believe that any rate of descent makes them susceptible to settling with power. I disagree.

 

1) Settling with power usually requires an airspeed at or below ETL, in addtion to the high rate of descent. The brisk walk maintains ETL to at least 50' if done properly. And hey, we have to slow down sometime.

 

2) A chart in Principles of Helicopter Flight page 160 (Also Rotary Wing Flight page 2-67) indicates that so long as an approach angle of 30 degrees or shallower should prevent SWP.

 

I'd love to hear any thoughts, comments, etc.

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I'm not a CFI, but I don't like more than 500 ft/min descent rate under normal conditions, and that's assuming it's a known landing area with no possible obstructions. If it's an off-airport landing, I keep it below 300 ft/min. I don't care about airspeed, I don't care about pitch attitude. I set up an approach angle depending on the area, and I keep that all the way down, and all I worry about is the angle and the rate of descent. I don't even look at the airspeed or the attitude, because I use whatever keeps me on the glideslope and on my descent rate. I've been using this technique for almost 40 years, landing to all sorts of areas, and it works well enough that I've never crashed or busted a checkride. I've used it day and night, to confined areas, offshore oil rigs, and to EMS scenes, as well as to airports. If you keep the descent rate under control, and the descent angle constant, you won't have problems with vortex ring state, settling with power, or anything else. The angle can be anything from 3 degrees to 90 degrees, and that depends on the situation. Night approaches into unprepared LZs demand a steep approach angle, to avoid hitting obstacles and wires, or at least give the best chance. An airport approach allows a shallower approach, which is easier and safer. The key is to maintain the angle from initiation to hover, and to keep the vertical speed the same all the way. Letting the descent rate build up too high is dangerous.

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From what I've read, you can make an approach at up to 30 degrees without having to worry about SWP. Up to that angle there are no VS-AS combinations that will get you into trouble. I calculated that at a 10-degree approach angle, approaching (initially) at 53 knots, which is the recommended initial approach speed in the 300, you have to have a 945 fpm descent to maintain the 10-deg. angle.

 

To make an approach at a 45-deg. angle, and maintain a 300 fpm descent rate, you would have to have a ground speed of about 3 knots. For a 30-deg. angle, about 5 knots. For a standard steep approach angle of 15 degrees, it would be an 11-knot GS.

 

The formula I've come up with is:

 

h = v / (110.2 * tan (a)); where (h) is the ground speed in knots, (v) is the vertical speed in fpm, and (a) is the approach angle in degrees.

 

For the airspeed, you would have to take your headwind component and airspeed calibration into account.

 

In summary, you can use any reasonable AS-VS combination that will get you there when approaching at an angle of 30 degrees or less. Over 30 degrees, then you need to limit your rate of descent. But keep in mind that judging your approach angle is only a rough guess; and whatever is possible is not necessarily safe.

 

Here's a way to judge your approach angle:

  1. Fly over a runway that has the solid white fixed-distance markers 1000' from the threshhold line. This must be done at your normal initial approach speed.
  2. Cross the threshhold at about 270 ft. AGL and see where the 1000-ft markers are on the windshield or some other suitable reference point. This is what a 15-deg. angle looks like.
  3. Do the same thing at 175 feet AGL for a 10-deg. angle, and about 90 feet for a 5-deg. angle.

 

Hope this helps,

 

~Jeff

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I'm not a student, but I would like to weigh in on one small detail. I'm 6'0. My primary instructor was 5'6. His "spot" on the -22 for a normal approach was much different than mine. As was his steep and shallow. Blanket statements like "halfway down the compass" can confuse someone with no prior experience who is just learning to shoot an approach.

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I'm not a student, but I would like to weigh in on one small detail. I'm 6'0. My primary instructor was 5'6. His "spot" on the -22 for a normal approach was much different than mine. As was his steep and shallow. Blanket statements like "halfway down the compass" can confuse someone with no prior experience who is just learning to shoot an approach.

 

 

I am glad that someone brought this issue up.. i noticed this when flying with different instructors. And then when you change to a different helo it changes as well... not a big issue, but as a new student, in a new helo, you have a lot of stuff to keep up with already!!

 

dp

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I'd like some CFIs to weigh in on the topic of normal and steep approaches, and their associated rate of descent. The way I teach my normal approaches is to maintain a descent angle using a sight picture. (Compass level on the 300C) Meanwhile, maintain an apparent brisk walk rate of closer. (Fast at 300', slower as you descend.)

 

Now, headwind component plays a big part in determining what the VSI will indicate as we fly this approach. In a NO WIND condition, the VSI may read upwards of 700 fpm. This makes some nervous, as they believe that any rate of descent makes them susceptible to settling with power. I disagree.

 

1) Settling with power usually requires an airspeed at or below ETL, in addtion to the high rate of descent. The brisk walk maintains ETL to at least 50' if done properly. And hey, we have to slow down sometime.

 

2) A chart in Principles of Helicopter Flight page 160 (Also Rotary Wing Flight page 2-67) indicates that so long as an approach angle of 30 degrees or shallower should prevent SWP.

 

I'd love to hear any thoughts, comments, etc.

 

The classic approach is flown at a constant angle with a more or less constantly decreasing airspeed and rate of descent to slow the the helo gradually, vertically and horizontally, throughout the descent. An approach that starts at 50, 60, or 70 knots must include a healthy initial rate of descent to fly the angle- I calculate 880 fpm at 50 knots and 10 deg (1300+ fpm for 15 deg 'steep' approach angle [airspeed management is more critical and challenging as the angle steepens]). That rate will gradually decrease, along with airspeed, as the approach progresses and you hold the selected angle. The idea is a gradual transition from altitude and high airspeeds with all rate changes constant, imperceptible and the visual point of reference fixed.

As long as you're above ETL, there's no exposure to settling with power- you're not maintaining position in your downwash as you descend.

The difficulty comes at the end of the approach with the loss of ETL and the visual reference starts moving down as you hold a constant angle to termination. The more or less 'large' power and control movements required at this point surprises pilots. This is the point where you start being exposed to SWP(settling with power) if you're behind the aircraft and allow it to stop horizontally, you add power for the loss of rotor efficiency and continue to descend as you attempt to land- the three things required to get into settling with power. Make the big control movements required to keep the helo moving, no matter how slowly, and you minimize the risk. The other major risk is the approach flown to fast with a big flare poorly controlled into zero movement across the ground, a honking power application to recover from the loss of ETL and continue descent- into SWP, too low to recover. Not too mention tail rotor strikes from that scenario. The old guy trick is patience and planning, making everything happen slowly and high enough so that one has time to do something when things go wrong- like an accidental downwind approach.

After loss of ETL, it's doubly important be patient and slow (The opposite of the initial impression of what's required to be safe) and keep the pig trotting along, or have a very stable power setting and S-L-O-W rate of descent while maintaining angle.

The final answer is whatever initial rate of descent required to establish the angle; carefully controlled airspeed, caution with airspeed and rate of descent after losing ETL and good power management. After I lose ETL, a ROD of 2-300 fpm. I want to do all this high enough that an abort isn't exciting.

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