Approach speed question

[CPA2FLY]

My CFII asked me why approach speed is 1.3so (90kts in the 172) and not say 1.5so. I didn't know the answer, but thought someone here could help me out.

Thanks.

 

[tgrayson]

My CFII asked me why approach speed is 1.3so (90kts in the 172) and not say 1.5so. I didn't know the answer, but thought someone here could help me out.

Hmmm....I think the stall speed, flaps up, power off for a C172R is about 51 knots, so 90/51 = 1.8, quite a bit larger than 1.3Vs0.

The approach speed is whatever you want it to be. 90 knots is common in the 172, because it's
1) only slightly larger than full flap operating speed
2) easy to work with mathematically (1.5 nm/minute)
3) has an entry in the timing tables on approach charts
4) keeps you in category A circling minimums

No reason to go slower, but you're welcome to go as fast as you like. But the faster you go, the less "think time" you have and your circling radius gets larger.

 

[Hacker15e]

I think he's asking why 1.3 Vso was picked as the approach speed across the board for all GA aircraft, vs something else like 1.5.

Was it an arbitary number, and if not, what were the reasons behind it?

 

[CPA2FLY]

Hacker, yes that is the question. Thanks.

 

[tgrayson]

I think he's asking why 1.3 Vso was picked as the approach speed across the board for all GA aircraft, vs something else like 1.5.

I still don't understand. In what way has 1.3Vs0 been picked across the board for all GA aircraft? For a VFR final approach speed, 1.3Vs0 is a good guestimate, but generally too slow for an instrument approach. The OP seemed to think that 90 knots WAS 1.3Vs0, which I pointed out was not the case.

As far as I can recall, the only significance that 1.3Vs0 plays in instrument approaches is the determination of the approach category. I'd be surprised if many actually fly them that slow.

 

[Italianaviator]

Hmmm....I think the stall speed, flaps up, power off for a C172R is about 51 knots, so 90/51 = 1.8, quite a bit larger than 1.3Vs0.

The approach speed is whatever you want it to be. 90 knots is common in the 172, because it's
1) only slightly larger than full flap operating speed
2) easy to work with mathematically (1.5 nm/minute)
3) has an entry in the timing tables on approach charts
4) keeps you in category A circling minimums

No reason to go slower, but you're welcome to go as fast as you like. But the faster you go, the less "think time" you have and your circling radius gets larger.

I think you got Vso confused with Vs1. Vso is stalling in the landing configuration. Which would be for the 172SP, 48X1.3=62.4 I think the published approach speed is 65 knots so its close enough. I know flight schools with large fleets of various types of aircraft want the approach to be flown at a faster speed (Embry Riddle 100 knots). Seminole's, Cessna's and Arrow's all fly approaches at 100 knots. And this is just so the traffic flow will be easier and smoother for ATC.

 

[wheelsup]

48X1.3=62.4 I think the published approach speed is 65 knots so its close enough. I know flight schools with large fleets of various types of aircraft want the approach to be flown at a faster speed (Embry Riddle 100 knots). Seminole's, Cessna's and Arrow's all fly approaches at 100 knots. And this is just so the traffic flow will be easier and smoother for ATC.

If ANYONE flies an instrment approach in a 172 @ 65 knots they should be shot.

"Published approach speed" would be like turning final 1 mile out. Not 10 on an approach.

 

[tgrayson]

I think you got Vso confused with Vs1. Vso is stalling in the landing configuration.

Well, I really didn't get them confused, but I did transition into talking about Vs1 because you don't fly instrument approaches in the landing configuration (light aircraft, anyway). Many people will use 10 degrees of flaps in the 172 (I do), so neither Vs1 nor Vs0 is completely appropriate.

Which would be for the 172SP, 48X1.3=62.4 I think the published approach speed is 65 knots so its close enough.

Yes, but the OP appeared to be talking about instrument approaches, rather than VFR approaches, because he mentioned 90 knots.

The original question is still unclear, because it's a little like "When did you stop beating your wife..."

 

[CPA2FLY]

tgrayson - I think your answer of "easy to work with mathematically (1.5 nm/minute)" is what my instructor was looking for. thanks.

 

[nosehair]

Category. He's talking about the approach speeds that you use to determine the approach category of your airplane to determine minimums.
The lowest speed you can use is 1.3 x Vs0 to determine if you are cat A, B, C, or D.

The 1.3 comes from a minimum safe speed above stall. You may notice the PTS for any checkride uses 1.3 time stall speed as a minimum short field approach speed if none is published in the POH.

30% above stall speed on a power off approach gives the necessary energy to effect a round out with no power.