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Old 14th Jul 2002, 10:02   #1 (permalink)
 
Join Date: Jul 2002
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Cessna 172 approach speed

I'll shortly be getting checked out in a Cessna 172 (1981 172P) and have been studying the Flight manual.
The indicated stalling speed in the landing configuration (Vs0) is given as 33 Knots but the short field approach speed is given as 61 knots.... a factor of around 1.85. I was always told that a short field approach was normally done at 1.3 x Vs0. Isn't 1.3 x Vs0 the normal Vref? Any ideas why the approach speed is so high?
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Old 14th Jul 2002, 16:46   #2 (permalink)
 
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I don't know the type, but I can think of a few reasons:-

(1) PEC mean that it isn't as high as 1.85 Vs
(2) The large drag flaps that I can't help noticing on a 172 may well permit a steep high speed approach, with speed bleeding off very rapidly in the flare, and that low speeds would jeapordise control in the flare. This is certainly broadly true of the C150, which I've flown a couple of times.


G
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Old 14th Jul 2002, 18:45   #3 (permalink)
 
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Thanks. Is the normal Vref of 1.3 x Vs0 based on IAS or CAS?
One thought that crossed my mind was that if the air speed indicator was to suffer from large errors at the lower end of the speed range then the 33 KIAS Vs0 may be well below the Calibrated stalling speed.
If this is the case then the 61 KIAS approach speed may actually be 1.3 x the Calibrated Vs0.
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Old 14th Jul 2002, 22:12   #4 (permalink)
 
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When you refer to the final approach speed from manual, is that the 50' / Vat speed? Or just a speed to maintain during the approach, with the intent to reduce to 1.3 Vs prior to the 50' point?

The C172s I've flown - up to but not including the P model - all had a Vat around 65 kts or less ie ~ 1.3Vs
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Old 14th Jul 2002, 23:26   #5 (permalink)
 
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Are you sure about that stalling speed? It sounds horribly low to me.

The short field Vapp sounds about right. For a normal field, in a C172 I'd aim for about 70-80kts across the fence.

You could, of course, experiment (I'd advise doing it with an instructor in tow until you're comfortable on the type) at around 3000 with stalling speeds.
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Old 15th Jul 2002, 02:58   #6 (permalink)
 
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Many years ago I instructed on the warrior, and the stall speeds were 50 clean and 45 with full flap. I doubt that a 172 would be much different.
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Old 15th Jul 2002, 03:02   #7 (permalink)
 
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I stick to 65kts... and if necessary, add a few knots for gusts etc.. or simply cause I am a bit sloppy sometimes...
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Old 15th Jul 2002, 06:33   #8 (permalink)
 
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Been flying 172's now for a while now and most recently the P model.

If you are doing a "normal" approach start on final at about 65kts and bring it back to 60kts over the fence.

For a "short field" approach you can be at 60kts on final bringing it back to 55kts over the fence.

If there are gusty conditions or strong winds add a few kts to the above but if it is calm or <5 or 10kts then that should do the trick. Even at 55kts for a short field approach you are still about 10 or so kts above the stall. Remember though that they do not have overly powerful engines so if you are heavy be ready with the power if needed.


EDITED TO ADD THIS BIT...

I forgot to add that if you keep the oil filled up to whatever figure is stipulated on the end of the dipstick then all you will ever get is the extra going straight out the breather. Next time you go to hop in one have a look under the cowl, near the fire wall on the pilots side and you'll see a bit of hose just sticking out of the fusulage. Thats where the extra will go. For example, if the stick say's 7Quarts, then if you have 6, or top it up to 6, you wont have a problem and your oil temps and pressures WILL stay in the green. Before I get nasty replies for this one - the manual does state that the aircraft is not to be operated with less than 5 Quarts.

This is straight out of a Cessna 172P manual

Quote:
White Arc - 33-85 KIAS Full flap operating range. Lower limit is maximum weight Vso in landing configuration.
So that figure is most certainly correct. Like I said, 55kts still gives you a nice buffer over the stall.

We plan on a block fuel of 33ltrs/hr and thats normally running at about 2400rpm with more often that not full loads.

Even thought the manual may not state it I've had mag fouling before because of aircraft has not being leaned under 3000' during previous flights (Obviously should really only apply on navs or longer flights - dont scare the sh*t out of your instructor by grabbing the mixture lever on downwind )

Hope some of this helps you,

Twin

Last edited by TwinNDB; 15th Jul 2002 at 06:57.
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Old 15th Jul 2002, 06:36   #9 (permalink)
 
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Northern Flyer,

Your second post is right on. From publication FAA-P-8740-49, p. 2: Calibrated airspeed should be used to calculate the proper approach speed at any landing weight and then converted to IAS for practical use. You should do this because, for some airplanes, the indicated airspeed near the stall has significant error.

How to do this: convert your IAS stall speed into CAS, multiply by 1.3 and convert that number from CAS into IAS. But it sounds like you figured all this out on your own. Very erudite.

This publication was written by William Kershner, a prodigious writer of aviation books in the U.S.
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Old 15th Jul 2002, 06:56   #10 (permalink)
 
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Airspeed relationships are always in terms of CAS. But, FAR-23 requires that the error full-range is never more than 5 kn, so unless it's 5 kn under-reading near the stall, and 5 kn over-reading on approach, it doesn't really explain why the published approach figure is so high.

Anybody here got any contacts at Cessna ?

G
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Old 15th Jul 2002, 07:37   #11 (permalink)
 
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Regulation FAR 25.1323 (c) states in part, “the instrument calibration error may not exceed three percent or five knots whichever is greater, throughout the speed range from Vmo to 1.3 Vs1 with flaps retracted: and 1.3 Vs0 to Vfe with flaps in the landing position.” I don’t know that it is the same for FAR 23 type aircraft.

From the 172 R manual, section 5.
Airspeed calibration for a normal static source (p. 5-9).

(The condition is power required for level flight or maximum rated RPM dive.)

Flaps Up KIAS/KCAS
50/56 60/62 70/70 80/79 90/89 100/98 110/107 120/117 130/126 140/135 150/145 160/154

Flaps 30 KIAS/KCAS
40/47 50/53 60/61 70/70 80/80 85/84

Stall speeds at 2450# (power off)
Rear CG flaps 0 : 10 : 30 (KIAS/KCAS)
44/51 : 35/48 : 33/47

Forward CG Flap 0 : 10 : 30 (KIAS/KCAS)
44/52 : 37:50 : 33/47

So using the above procedure, stall is 33 indicated which equals 47 calibrated, multiplied by 1.3 is 61 KCAS which from the above table is 60 KIAS. The short field landing distance chart (the only landing distance chart supplied) has a condition of 62KIAS at the 50 foot height. Obviously you would choose this method (or not) when other numbers aren't published.

If you had good data (which Cessna hasn't published) you would have KIAS vs KCAS for power off also. In the above I made the assumption that at 60Kts the power-off would be similar to the power-on. The FTE's can enlighten us if this is a poor assumption.

Last edited by '%MAC'; 15th Jul 2002 at 09:50.
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Old 15th Jul 2002, 09:04   #12 (permalink)
 
Join Date: Jul 2002
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Thanks for the replies. The flight manual gives a flaps down, power off calibrated stalling speed of 46 knots. As mentioned before, the Vs0 or bottom of the white arc is 33 KIAS. The landing distance table (short field) is based on a speed of 61 KIAS at 50'.
Comparing the 33 KIAS Vs0 of the 1981 172P to older models (172N's etc) I've noticed that this Vs0 is very low. I assume this could simply be due to a different model of ASI or pitot static system used... any other ideas?
Many flight training books quote short field approach speeds of 1.3 x Vs0 but fail to mention this is based on CAS. With such a difference in Calibrated and indicated Vs0's I believe Cessna should mention this in their manual.
A student may be tempted to reduce the approach speed because of the big margin above the stall on the ASI. For example, 1.3 x 33 KIAS = 42.9 Knots. Using the airspeed calibration table this would give a CAS of around 49 Knots... only 3 knots above the Calibrated Vs0!!! Worth a thought.
(The airspeed calibration table states: 'condition: Power required for level flight or maximum rated RPM dive' - I suppose these speeds may also be different with power off!)
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