Speed?

CE402

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Does anybody know how the FAA defines speed with regards to Vso? The definition in Part 1 just says "the stalling speed or the minimum steady flight speed.....etc." without specifying IAS or CAS. It sounds like I'm splitting hairs, but it can make a big difference when 1.3 x Vso is right on the line between to approach categories.
Thank you in advance.
 

bigsky

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I am pulling this from part 25; 103 which is " airworthiness standards: transport category"
For Vs( not Vso, but I am sure it is the same)
Vs is the calibrated stalling speed or the min steady flight speed, in knots....
 

CE402

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Thanks

I greatly appreciate the answer, complete with reference. That one's been bugging me for a while.
Safe skies...
 

avbug

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The simple answer is that it depends on the aircraft. Some aircraft define speeds in calibrated airspeed, and then provide conversion charts. Some define performance in terms of indicated airspeed.

The FAR makes reference only to speed, in knots. If your performance is given in CAS, and this is your reference for Vso, then use CAS. If it's given in KIAS, then use indicated airspeed.

Approach categories are predicated on the potential performance of the airplane, and apply specifically to obstacle clearance protection for circling, and for the missed approach. The clearance criteria is determined based on the potential turning radius of the aircraft at the upper end of each approach category.

It is for this reason that aircraft circling at higher speeds, should use the next higher category.

Note that the clearance is based on speed over the ground; not airspeed. One cannot predict the potential for changes in wind velocity. When obstacle protection is given for circling at category C, for example, radius is computed at a maximum ground speed of 141 knots. An aircraft which is a category C aircraft, but circling at 150 knots, should clearly use category D.

However, a category C aircraft which is circling at 135 knots, with a 15 knot tailwind, is going to exceed the circling maneuvering area for obstacle clearance, and should still use category D minimums for protection. A good rule of thumb is that if indicated airspeed, or ground speed is high enough to warrant the next higher category of approach minimums, then use those minimums. This isn't an issue when landing straight in, but is very important when circling, and when executing a missed approach. Remember that the missed approach is predicated on being initiated at the MAP, established according to the minimums for the category of approach in use. Higher minimums may be required to allow for a wider turn radius on the missed.

Speed may be IAS or CAS, depending on the airplane. However, for cockpit reference, all speeds should be converted to IAS for use.
 

CE402

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Thank you AVbug

Your detailed answer is much appreciated. The problem arose when I discovered that the performance supplement for the VG STC on our 402s lists the stall speeds in CAS. The standard AFM for the VG-less aircraft lists stall speeds in both CAS and IAS. 1.3 x Vso, without the VGs, puts the aircraft into approach category B if you are using IAS, but it remains category A if you use CAS. Hence the confusion. The only other publication I could find to support the CAS argument, besides waht Bigsky found, was a FAA "Flight Safety Publication" called "On Landings" . I found that one after looking through LOTS of documents. SOmetimes this stuff is as clear as coffee. Take care.
 
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avbug

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The bottom line is safety. The safety of your flight depends on obstacle clearance, and this is predicated on the actual speed of the aircraft over the ground. Not calibrated speed, not airspeed, but actual groundspeed. In still air near sea level, IAS will closely approximate ground speed. At higher elevations and on hot days in still air, TAS will be higher, and subsequently your groundspeed will be higher. You will use more room getting turned around; the radius is bigger, and therefore you should use the higher category. This is the intent of requiring you to use the next higher category when circling.

If there is any question or your're close, move up to the next category. Safety is cheap. Be conservative.
 
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