PTS Approach question

[Jaybird180]

Reading the PTS it states that approach to landing should be 1.3 VS0, which in the C172 would be at ~43 kts. POH approach is 65 kts and I've been trained at 60kts. POH states that short field is 51kts.

Help me make sense of this.

 

[poadeleted20]

1.3 Vs0 is computed based on Calibrated Air Speed (CAS), not Indicated Airspeed (IAS). The published 33-knot full flap stall speed of a 172 is IAS, not CAS. Go to the airspeed calibration table in your POH, and you'll find that an IAS of 33 knots flaps down is a CAS of about 46 knots. Multiply 46 knots by 1.3, and you get 60 knots (well, 59.8, but that's 60 to me on the airspeed indicator), which is a good max gross final approach speed with full flaps in a 172 (60 CAS being just about 60 IAS).

 

[jnmeade]

Multiply 46 knots by 1.3, and you get 60 knots (well, 59.8, but that's 60 to me on the airspeed indicator), which is a good max gross final approach speed with full flaps in a 172 (60 CAS being just about 60 IAS).

Sorry, I'm not following. Can you say it another way and I'll try to get my head around it?

 

[poadeleted20]

Sorry, I'm not following. Can you say it another way and I'll try to get my head around it?

The FAA's recommendation for 1.3 Vs0 is all based on CAS, not IAS. However, the flaps down stall speed you see on the airspeed indicator (bottom of the white arc) is IAS, not CAS. To get the "right" speed on final, you have to start with Vs0 in CAS, not IAS, and you get that from the POH stall speed table. Then, multiply the Vs0 in CAS by 1.3 to get the recommended CAS for final approach. Finally, use the airspeed calibration table to convert the final approach CAS to an IAS to shoot for on the airspeed indicator.

In the case of the 172, there's a huge difference between CAS and IAS near the stall (46 CAS vs 33 IAS), and that's what's driving the OP's problem (using 1.3 x 33 = 43 instead of 1.3 x 46 = 60). However, at the "good" approach speed of 60 knots CAS, there's not much difference between CAS and IAS, in contrast to the large difference at the flaps down stall speed.

 

[Jaybird180]

Don't have POH in front of me right now, but I had't realized that the ASI was that far off. They must do that on purpose to keep students current with reading performance charts and whiz wheels.

.....ahhhhgggggggg...now you just messed up my head, gotta look at it when I get home...don't remember if all the performance numbers are IAS or CAS...........

 

[poadeleted20]

Don't have POH in front of me right now, but I had't realized that the ASI was that far off.

It is in a 172 near flaps down stall speed.

They must do that on purpose to keep students current with reading performance charts and whiz wheels.

I will not attempt to fathom motivations, although I suspect it's an engineering problem of balancing priorities.

.....ahhhhgggggggg...now you just messed up my head, gotta look at it when I get home...don't remember if all the performance numbers are IAS or CAS...........

As I said above, I recognize that 43 knot figure as being 1.3 times Vs0 IAS from other folks with the same question. I just hope I didn't bust your balloon thinking you were the first Student Pilot to discover this issue.

 

[Trapper John]

.....ahhhhgggggggg...now you just messed up my head, gotta look at it when I get home...don't remember if all the performance numbers are IAS or CAS...........

Cessna is good about telling you whether the number they show is IAS or CAS (or TAS in the performance charts).

 

[Palmpilot]

Don't have POH in front of me right now, but I had't realized that the ASI was that far off. They must do that on purpose to keep students current with reading performance charts and whiz wheels.

I suspect that when airspeed indicators were introduced, the technology of making them reflect reality more accurately would have been prohibitively expensive.

 

[jesse]

Reading the PTS it states that approach to landing should be 1.3 VS0, which in the C172 would be at ~43 kts. POH approach is 65 kts and I've been trained at 60kts. POH states that short field is 51kts.

Help me make sense of this.

The PTS states:

5. Maintains a stabilized approach and recommended airspeed, or in its absence, not more than 1.3 VSO, +10/-5 knots, with wind gust
factor applied.

It doesn't say 1.3. It says 1.3 if there is no manufacturer recommended airspeed.

 

[bbchien]

Jay, there is a table/graph in the back of yor POH that shows the divergence of IAS from the TAS as the flaps go down and the airspeed decreases. It's not a little deal.

 

[elmetal]

Jay, there is a table/graph in the back of yor POH that shows the divergence of IAS from the TAS as the flaps go down and the airspeed decreases. It's not a little deal.

Yep

When I first learned in ground school about KIAS, KCAS, KTAS I was just like WTF USE ONE SPEED!!!!!!!!!!!!!!!!!!!!!!!!!

Then I learned what each one was, and honestly never though about KCAS again until I started learning about stalls in the plane and Vso and Vref until I realized in the POH that at low and slow speeds, specifically with the flaps down (aka high angle of attack more than likely) the KCAS was vastly different from the KCAS.

but I always try to think in KIAS since KCAS is pretty useless to me when I'm sitting in the plane looking at the airspeed

 

[gismo]

Yep

When I first learned in ground school about KIAS, KCAS, KTAS I was just like WTF USE ONE SPEED!!!!!!!!!!!!!!!!!!!!!!!!!

Then I learned what each one was, and honestly never though about KCAS again until I started learning about stalls in the plane and Vso and Vref until I realized in the POH that at low and slow speeds, specifically with the flaps down (aka high angle of attack more than likely) the KCAS was vastly different from the KCAS.

but I always try to think in KIAS since KCAS is pretty useless to me when I'm sitting in the plane looking at the airspeed

You need to use IAS for target speeds on the ASI. But for ANY computation you need to use CAS since that's what really matters but then you've gotta convert back to IAS to use it in the airplane. In most airplanes the difference is small enough to ignore above Vy.

FWIW, there are several things that make IAS different from CAS and they all vary from one airplane model to another. Some examples are static system errors due to pressure buildups, angle between incoming air and pitot, and indicator errors at low pressure.

 

[TMetzinger]

To reinforce what Jesse said - whenever there's a difference between the "general" procedures in the airplane flying handbook or PTS, and the "specific" procedures in an airplane's documentation (AFM/PIM/POH), you follow the airplane documentation.

In this specific case, the difference between IAS and CAS makes it seem like there's a difference, but as Ron notes, there really isn't a difference in the actual speed the plane is flying through the air.

Bonus question for the student - what do you think might cause the wide discrepancy between the IAS and CAS at the slower speeds?

 

[poadeleted20]

To reinforce what Jesse said - whenever there's a difference between the "general" procedures in the airplane flying handbook or PTS, and the "specific" procedures in an airplane's documentation (AFM/PIM/POH), you follow the airplane documentation.

That's what the PTS says. However, if you follow the Cessna POH for 65-70 knots on final for a 172, you're not going to be able to avoid a significant float, especially at typical training/testing weights, and (as discussed to no end here and on other sites) landing becomes significantly more difficult. How you choose to deal with that conundrum is your business, but I've never heard of an examiner busting an applicant solely because the applicant flew a 172 at 1.3 Vs0 on final rather than at 65-70 knots.

 

[Jaybird180]

Bonus question for the student - what do you think might cause the wide discrepancy between the IAS and CAS at the slower speeds?

WAG: At slow speed and flaps down this creates an increased pressure at the static port, thereby affecting the reference for the leading edge mounted pitot.

 

[Jaybird180]

Thanks for pointing out that nuance, Jesse. I missed it.

 

[TMetzinger]

WAG: At slow speed and flaps down this creates an increased pressure at the static port, thereby affecting the reference for the leading edge mounted pitot.

You ARE thinking about the right system and understand that the airspeed indicator is driven by the difference in pitot and static pressure.

The problem we have is that the ASI is reading "low", right? The IAS is less than the CAS. It could be that somehow static pressure is raised at the static port, but isn't it more likely that ram pressure is reduced at the pitot? What would be the explanation for reduced ram pressure at the pitot?

 

[elmetal]

You ARE thinking about the right system and understand that the airspeed indicator is driven by the difference in pitot and static pressure.

The problem we have is that the ASI is reading "low", right? The IAS is less than the CAS. It could be that somehow static pressure is raised at the static port, but isn't it more likely that ram pressure is reduced at the pitot? What would be the explanation for reduced ram pressure at the pitot?

I think the ASI reads high, so when ASI = 45knots near stall (full flaps), the CAS is closer to 35 isn't that right? I don't have any sort of POH here at work

 

[TMetzinger]

I think the ASI reads high, so when ASI = 45knots near stall (full flaps), the CAS is closer to 35 isn't that right? I don't have any sort of POH here at work

No, you have it backwards according to Ron and the table Bruce posted... See above. The ASI reads "low".

 

[poadeleted20]

No, you have it backwards according to Ron and the table Bruce posted... See above. The ASI reads "low".

...at the low end, usually (although I know of no case to the contrary). As you can see from that table, it may read high at the high end.

 

[Jaybird180]

Thanks for putting my knowledge (or ignorance) out in the public for all to see, you're a real pal Ted (LOL Good question, BTW)!

While I'd considered that the pitot pressure was lower than it should be, I reasoned that considering the relative position of the two, it was more likely that the static pressure was high.

However, at your suggestion that my answer was very near correct, I would reason that the manufacturer understood that they had to choose where to calibrate the ASI for IAS=TAS, that factor being mostly influenced by AoA, which has a direct impact on speed. They chose somewhere near cruise. This is also the reason why a few aircraft have a movable pitot, controllable by flap (and/or slat) position (I was on a commercial flight and saw this once - don't recall the aircraft type though)

But this still doesn't answer the question as to why the pitot pressure is low. For this, I'm not satisfied that Bernoulli's principle (the pitot acting as a venturi) is the culprit. So, I'll do some research online (BTW- This thread shows up in a Google Search -LOL) and get back to ya.

 

[poadeleted20]

The pitot pressure is low at low speeds/high AOA's because of the high angle of the pitot tube to the relative wind, with losses due to the air trying to "go around the corner" and then down the tube.

BTW, my suspicion is that this effect will also occur at high AOA's even at high speeds, say, in a 60-degree bank level turn pulling 2 g's while trying to stay below maneuvering speed, but I've never seen anything on point.

 

[GeorgeC]

I would think the difference stems from a smaller component of the relative wind going into the pitot due to high AoA.

 

[Jaybird180]

The pitot pressure is low at low speeds/high AOA's because of the high angle of the pitot tube to the relative wind, with losses due to the air trying to "go around the corner" and then down the tube.

BTW, my suspicion is that this effect will also occur at high AOA's even at high speeds, say, in a 60-degree bank level turn pulling 2 g's while trying to stay below maneuvering speed, but I've never seen anything on point.

Isn't that called a zoom? Doesn't exist in the world I live in.

 

[poadeleted20]

Isn't that called a zoom? Doesn't exist in the world I live in.

I guess a steep turn is a bit of a "zoom" in that you're pulling g's, but you're doing the zooming mostly in the horizontal rather than the vertical.

To pull 2 g's in the turn, you have to produce lift equal to twice the weight of the plane. That means either doubling coefficient of lift, or increasing speed 41%, and your 172 doesn't have enough power to increase speed that much. As a result, you have to increase coefficient of lift, which takes more AOA, and creates more drag, slowing you down further.

When you stabilize, you'll probably be around 90 KIAS, with at least cruise power, and an AOA maybe 6 degrees higher than in normal cruise flight. While that's not nearly as much more than stalling AOA is over normal cruise, I'm guessing that's enough to create the effect which leads to IAS being less than CAS, just not as big a difference as at stall.

 

[Jaybird180]

2G turn is also at 60deg bank but ....

I was hoping you would elaborate on high AoA and high speed at the same time. Seems to defy what I've been taught about AoA, speed, stall tendencies, etc.

 

[poadeleted20]

Very simply, the more g you pull at a constant speed, the more AOA it takes to get the extra lift. Typically, you're only at a few degrees of AOA in normal cruise flight, but when you pull 2 g's, you may be up close to 10 degrees at a much higher speed than you'd be needing 10 degrees AOA for 1-g flight (down in the "slow flight" regime).

 

[Jaybird180]

Thanks for putting my knowledge (or ignorance) out in the public for all to see, you're a real pal Ted (LOL Good question, BTW)!

While I'd considered that the pitot pressure was lower than it should be, I reasoned that considering the relative position of the two, it was more likely that the static pressure was high.

Okay, that doesn't make sense....That would totally screw up the altimeter and VSI!!!

 

[TMetzinger]

Thanks for putting my knowledge (or ignorance) out in the public for all to see, you're a real pal Ted (LOL Good question, BTW)!

While I'd considered that the pitot pressure was lower than it should be, I reasoned that considering the relative position of the two, it was more likely that the static pressure was high.

However, at your suggestion that my answer was very near correct, I would reason that the manufacturer understood that they had to choose where to calibrate the ASI for IAS=TAS, that factor being mostly influenced by AoA, which has a direct impact on speed. They chose somewhere near cruise. This is also the reason why a few aircraft have a movable pitot, controllable by flap (and/or slat) position (I was on a commercial flight and saw this once - don't recall the aircraft type though)

But this still doesn't answer the question as to why the pitot pressure is low. For this, I'm not satisfied that Bernoulli's principle (the pitot acting as a venturi) is the culprit. So, I'll do some research online (BTW- This thread shows up in a Google Search -LOL) and get back to ya.

You got it!!!! as Ron notes, the AOA between the pitot and the relative wind means a reduction in pressure in the pitot.

And my original response to you was intended to let you know that you almost got it right - and to encourage you to keep thinking about it without giving you the answer. I certainly didn't mean to embarrass you.

 

[Jaybird180]

No embarrassment taken, its tough being sarcastic online.