IAP airspeed - IAS or TAS??

[wrxpilot]

Let's say you're flying a VOR appch at a high altitude airport in Colorado in something like a 172, so your appch speed will be something around 90 kts (category B). The right way to do this would be to get your ground speed to 90 kts. Assuming no wind (TAS = GS), your IAS might be somewhere around 75 kts (25C, 8000 ft).

So what's the real world way of flying this, assuming no DME or GPS to show your groundspeed? Do you fly in at a known pwr setting for some speed (let's assume 90 kts IAS), grab your flight computer, and make a best guess on winds to determine ground speed? Then you'd have to make a best guess at interpolating the category/time table.

It just seems like too much to worry about during the heat of an appch. Obviously an appch using DME or radials for stepdowns and MAP is much better. But I'm just curious to see what more experienced types do if they're using time.

 

[Fblowjets]

Put all that geewhiz stuff away and fly the plane..

 

[MauleSkinner]

Let's say you're flying a VOR appch at a high altitude airport in Colorado in something like a 172, so your appch speed will be something around 90 kts (category B). The right way to do this would be to get your ground speed to 90 kts. Assuming no wind (TAS = GS), your IAS might be somewhere around 75 kts (25C, 8000 ft).

On what are you basing the "right way to do this"? Note that the AIM doesn't specify TAS for the approach categories, rather a percentage of the stall speed:

5-4-7. INSTRUMENT APPROACH PROCEDURES
{New-2002-4 5-4-7a Revised February 21, 2002}
a. Minimums are specified for various aircraft approach categories based upon a value 1.3 times the stalling speed of the aircraft in the landing configuration at maximum certificated gross landing weight. In 14 CFR Section 97.3(b) categories are listed as follows:
1. Category A: Speed less than 91 knots.
2. Category B: Speed 91 knots or more but less than 121 knots.
3. Category C: Speed 121 knots or more but less than 141 knots.
4. Category D: Speed 141 knots or more but less than 166 knots.
5. Category E: Speed 166 knots or more.

Fly safe!

David

 

[wrxpilot]

On what are you basing the "right way to do this"? Note that the AIM doesn't specify TAS for the approach categories, rather a percentage of the stall speed:


Fly safe!

David

I'm basing it on the tables in appch plates that reference GS (Ground Speed). Read my question again (hint, it's not about the definition of category). If your GS is totally off, then timing for a appch is totally useless.

Here's what got me thinking about this in the first place:
http://forums.flightinfo.com/showthread.php?t=83812

And the IAP for that:
http://204.108.4.16/d-tpp/0608/05574V13.PDF

I was looking at that plate thinking it would sure suck if your GS was a lot different than you thought (assuming you can't identify CAM with DME).

 

[wrxpilot]

Put all that geewhiz stuff away and fly the plane..

So, in other words all that 180 hr private pilot instrument talk went right over your head. Ok... Do us a favor and stay out of the training section and keep your comments to gojets bashing in the regionals or whatever it is you do.

 

[Stifler's Mom]

I just base any timing of an approach on my indicated speed. You'd be suprised that at 2000' you might have a 20kt tailwind that shears into a headwind as you descend. However, you'll probably be close on your timing with IAS.

 

[JessMan]

I second the IAS, and most commercial operators are going to have a GPS or at least a DME to display your ground speed. In any condition you'll have a reference speed plus a fudge factor (ref+10 knots at a minimum in my plane) and in windy conditions whomever is flying with ask the PNF what the groundspeed is doing.

 

[avbug]

Minimums and obstacle clearance are predicated on your actual speed. Calculate it in advance, before you fly; make it part of your preflight planning. If you know what the winds are, simply add them to the calculation you made before the flight (unless temperature has changed drastically).

Basing your approach category on your ground speed is always a better idea than merely shoehorning into the category for your aircraft. If you're circling, the circling radius for obstacle protection is based on groundspeed, not indicated airspeed, and must be used. Regardless of w(h)eather you circle, use the approach category and attendant minimums based on what's really occuring, not what you're indicating in the cockpit. When in doubt, bump up to the next category.

 

[MauleSkinner]

I'm basing it on the tables in appch plates that reference GS (Ground Speed). Read my question again (hint, it's not about the definition of category). If your GS is totally off, then timing for a appch is totally useless.

Here's what got me thinking about this in the first place:
http://forums.flightinfo.com/showthread.php?t=83812

And the IAP for that:
http://204.108.4.16/d-tpp/0608/05574V13.PDF

I was looking at that plate thinking it would sure suck if your GS was a lot different than you thought (assuming you can't identify CAM with DME).

Maybe I've still got a different idea about the question, but here's what I'd do on this specific approach...I'd still fly my approach at 90 KIAS, if that is appropriate for my airplane. Technically, I could use Cat A minimums, but since it's right on the margin, I'm going to use Cat B minimums (they're the same anyway). As Avbug indicated, don't try to shoehorn your airplane into the approach...If I were flying an airplane that was too close to Cat D speeds, I wouldn't shoot this approach.

As you indicated, timing is based the "FAF to MAP" table, on which posted speeds are GS. You need to calculate your TAS and at least estimate the winds you will be dealing with.

If you had 30 knots of tailwind instead of "no wind", you would be looking at going missed about 2 miles beyond the MAP. The way I understand it, the 10nm ring on the chart indicates an area inside which obstructions are to scale. Looks to me like as long as you're not one who hangs around at minimums a few seconds more, just in case there's a break, you're still going to be initiating your missed approach turn in time to clear obstacles.

Assuming a descent rate to minimums of 7-800 ft/min, you're looking at just over 2 minutes, leaving at least a minute and a half AT minimums...a minute is a LONG time to be at minimums and not see anything. In an approach like this, if you're still solidly in the clouds, initiating the missed approach a few seconds early would probably resolve any "DME doubts" you may have, although technically you should make your missed-approach climb straight ahead until you've gotten to your calculated time.

You might also take a look at the TERPS, to find out how the obstacle clearance criteria are established for a timed approach versus one that requires DME...you might find it somewhat reassuring as well.

Fly safe!

David

 

[A Squared]

I'm basing it on the tables in appch plates that reference GS (Ground Speed). Read my question again (hint, it's not about the definition of category). If your GS is totally off, then timing for a appch is totally useless.

Here's what got me thinking about this in the first place:
http://forums.flightinfo.com/showthread.php?t=83812

And the IAP for that:
http://204.108.4.16/d-tpp/0608/05574V13.PDF

I was looking at that plate thinking it would sure suck if your GS was a lot different than you thought (assuming you can't identify CAM with DME).

well, to address the question in the context of that approach: the conditions quoted in the newspaper for the time of the accident would have resulted in an TAS about 5% greater than the IAS, or .3 nm over the 6 nm timing segment. , or an error of a little over half the runway length, or more relevantly, .3 nm past the MAP would still be 1 nm before the runway threshold.


Something a lot more serious than a 5% error in groundspeed happened to put him into the mountain on the far side of the airport.