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3 Answers

Flying by the Numbers: C-172SP

Asked by: 7541 views Aircraft Systems, Instrument Rating

What would your recommendations be for a C-172SP (180 HP) to fill out the following table:

(columns) RPM, Pitch Setting, Airspeed, and VSI

for each of  (rows): Climb, Cruise, Cruise Descent, Approach, Approach Descent, Non-Precision Descent

I am trying to get a consistent set of numbers for IFR training. The POH is some guide, but beyond that seems open to interpretation. Just wondering if I can find a consensus. Thanks.

3 Answers

  1. Brian on Jan 19, 2012

    Climb: Full power and pitch for Vy. Cruise climb: Full power, lean as needed, pitch for any airspeed that you can still maintain 500 fpm climb with. Altitude will determine this speed. At 3,000 feet you can probably do it at 100 knots, at 10,000 feet you probably will need a Vy climb just to maintain 500 fpm.
    Cruise: As needed for planning, use your POH. There is no one right answer, it will be dependent on mission profile/needs.
    Cruise Descent: As needed for planning. Again there is no right answer. Sometimes you’ll have to lose 2,500 in 5 miles, sometimes 5,000 in 8 miles. You’ll need to decide your desired descent rate based on forward speed (see last paragraph), distance, and how far you need to descend.
    All Approaches: I always used between 1800-2000 rpm with pitch setting necessary to maintain 90 knots with 10 degrees of flaps once on the inbound leg inside the FAF. Before the FAF is just like the cruise descent, it will change with every flight condition, thus no one answer can be given (again see last paragraph).
    A few helpful rules of thumb:
    Target vertical speed for a 3 degree glide slope is ground speed x 5. So if you’re going 90 your target vertical speed is 450 fpm. Adjust power as necessary to attain this and you can practice such at altitude.
    Distance from touchdown on a 3 degree glideslope in miles is AGL/300. In other words, if you’ve captured the glide slope at 3,000 feet AGL / 300 = 10 miles from the airport.
    90 knots ground speed is 1.5 miles a minute and 120 knots ground speed is 2 miles a minute. Use these as target speeds for all planned descent estimates to make the math easier. You can always interpolate for 110/130 etc, but that’s more challenging. 

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  2. Kent Shook on Jan 25, 2012

    That table should be filled out in flight, in the airplane you’re flying for your training. The point is so that you know the correct power settings for each phase of flight so that you can get the performance you desire without needing to constantly fiddle with power. That way, you’ll be able to concentrate the other things you need to concentrate on for instrument flying. Since every airplane performs somewhat differently, copying numbers out of a POH or from a web site such as this one isn’t going to have the desired effect.

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  3. Curtis Ide on Jan 25, 2012

    Brian did a good job of expanding and Kent is correct that you will need to record data for your specific airplane in flight.  I have a couple things to add that may help as well.
    Typical 172 information and what you will be looking for:
    Climb – Climbs will be done at full power generally at Vx or Vy but may also be done at constant rate.  I generally familiarize my students with 500fpm constant rate climbs.  I would suspect you will be near 6-8 and even up to 10 degrees or so nose up at full power for Vy but you can record the pitch setting once you obtain the proper airspeed or vertical speed.   
    Cruise – Use book settings for cross country flights.  Otherwise typical power settings for manuevers will be 2100 and 2300 rpm.  Consult with your instructor for what he specifically uses in that plane.  I have found over the years that in most small Cessna models you can expect about 5 kts of airspeed change per 100 rpm i.e. 2300 rpm may be 115 kts making 2200 rpm 110kts.
    Cruise Descents – These can be done at a wide variety of airspeeds and vertical speeds so you will have to practice with this.  Generally if you are doing a true cruise descent if you are cruising at 120kts and want to descend you would leave the trim alone (because you are trimming for airspeed) and slowly reduce the power to obtain 120kt descent at 500-1000 fpm depending on your target.
    Approach – Generally I like to be at a slow cruise speed that allows me to put flaps down to the desired setting for the the approach.  Different instructors have different thoughts on this so you will need to consult your instructor.  I typically have been flying 172N,K, and R models which are a little different although I have some in the 172SP.  I would expect you will want to fly between 80-90 kts which may be around 1900rpm in the SP.
    Approach Descent –  Brian was dead on about predicting your desired vertical speed – this is very important for situational awareness and staying ahead of the airplane.  For the 3 degree glide slope on most ILS systems you can assume ground speed X 5 or I like to say ground speed / 2 and add a zero for the same result i.e. 90/2 =4.5 or 450.  On an ILS I typically employ full flaps at the final approach fix which should put you at about 65-70 kts on the ILS and a 350fpm descent at about 1500-1700rpm.  On non-precision approaches or circling approaches you can expect to use 10 degrees of flaps and about 800 fpm descent depending on what kind descent is needed.  The FAA doesn’t want to much dive and driving anymore so don’t use an excessive descent rate to get down really early on the non-precisions – i.e. no more that 1000fpm.  You may also ellect to use full flaps on some non-precision straight in approaches particularly now with having vertical guidance on non-precision GPS approaches.
    Side Note: Everyone uses different flap settings on approaches and this is always a point of discussion.  I like to use full flaps on ILS approaches because I want to be stabilized and in full landing configuration prior to being low.  In complex airplanes this means gear down/flaps down/all other systems set.  It’s not a good practice, in my opinion, to shoot an ILS to minimums at 200 ft agl see runway lights and descend to 100 ft agl and then decide to put more flaps in to land.  On non-precision approaches you may have to level off which is why i don’t use full flaps.  Once making the final descent to landing on the non-precision I will add the remaining flaps.  However, even on non-preicision approaches all of my other landing checklist are complete prior to the final approach fix. 

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