Short-field approach speed vs normal approach speed

[Raskolnikov]

I've noticed at least a couple of airplanes manufactured in the 70's that list a normal approach speed as a range and the short field approach speed as a specific number. Early 1970's 172 lists normal approach as 65-75 MPH (flaps 40) and the short field approach as 70 MPH (flaps 40).

Why wouldn't they recommend the short field approach speed as 65 MPH, the bottom of the normal range?

 

[Pucker Factor]

1.3Vso should work just fine.

 

[Raskolnikov]

Ah yes, Vso is 54 MPH making 1.3*54 = 70.2 MPH
But why would they place the bottom of the range for normal approach below 1.3 Vso?

 

[Pucker Factor]

I guess is to help with all the drag in a full flap configuration. As I'm sure you've found out. With full flaps as soon as you pull power you fly like a brick.

 

[Raskolnikov]

The flying like a brick part is what got me thinking about this. If you approach at 65 MPH and pull power over your aim-point you can really keep the ground roll to a minimum. At 70 MPH you'll roll a little bit farther down the runway.

I'm teaching short field approach speeds as published (70 MPH), but want to have a good explanation ready for what looks like a conundrum.

I'm getting back into instructing after about a three year break from flying. So am a bit rusty on some things.

Appreciate the thoughts / comments.

 

[flyingbum]

The old Cessna's have some shotty POH's. As a matter of fact, I have a Cessna POH that contradicts itself on Vx and Vy speeds. Pre-mid 70's POH's weren't as standardized as they are today. I think the info was more dependent on test pilot data vs. average piloting technique. 1.3 of Vs0 sounds good. But don't forget, Vs0 was sometimes based on 20 degrees flaps, not 30 or 40 degrees.

 

[shdw]

Ah yes, Vso is 54 MPH making 1.3*54 = 70.2 MPH
But why would they place the bottom of the range for normal approach below 1.3 Vso?

Are you using CAS for your 1.3 Vso calculation? If not, you should be. Calculate in CAS then convert to IAS so as to have a useful number to reference in the A/C.

 

[MidlifeFlyer]

In addition to what has already been mentioned, keep in mind that the recommended speeds (especially the range) are loose estimates based on the aircraft being at max gross weight. And those numbers are so close together in something as small and light as the typical trainer that it really doesn't make much difference. So, it's fine to use the POH numbers for training-to-the=checkride purposes but really calculating 1.3 vs0 for normal landings and 1.2 Vs0 for short field landings would give your students some useful knowledge for the future.

Rasko, explaining that to my students is how I deal with the "conundrum."

 

[Blackhawk]

Just to add to what Mark wrote.
The formula used for figuring Va at lower weights can also be used to figure your short field approach speed at lower weights if the POH only gives one number:

Actual speed=sqare root of OW/MGW x book speed.

So, for the old 172 using 65 MPH (I think 70 is too fast), if you are operating at 2000 lbs instead of 2300 lbs your speed would be square root of 2000/2300x65= 60.61 MPH.
I would check with an examiner before using the above in a check ride. Some like it because it is real world, others don't because it is not in the PTS.
Also, PTS does not state to use 1.3xVso, but "...not MORE than 1.3x Vso +10/-5...". So I guess using the formula above you would be legal per the PTS as your speed would be not MORE than 1.3 x Vso. But again, while i teach this to all my students I check with DEs before having them use it in a check ride.

 

[Raskolnikov]

Fantastic! Thanks everyone!

 

[ppragman]

I've noticed at least a couple of airplanes manufactured in the 70's that list a normal approach speed as a range and the short field approach speed as a specific number. Early 1970's 172 lists normal approach as 65-75 MPH (flaps 40) and the short field approach as 70 MPH (flaps 40).

Why wouldn't they recommend the short field approach speed as 65 MPH, the bottom of the normal range?

The amount of energy you have on touchdown is the amount of energy you have to get rid of in the roll-out. Remember that the energy goes up by the square of your speed - meaning double the speed and you quadruple the energy. Remember also, stall speed varies with weight, power setting, flap setting, ground effect, and that's the "slowest possible touchdown speed" where you're in control. Depending on how short the strip is, and how comfortable you are, an approach speed of around 1.1Vso may be better suited in the last part of the approach. I'd often have two power settings in my head, the one that would give me 1.3 Vso - ish, and the one that would put me right above stall and feel it on the rest of the way from there and transition to this second power setting around 200' AGL. I remember in the 206 coming in with just enough airspeed left to be able to able to flare and feeling the stall buffet an instant before the mains touch - that's the speed to have the shortest possible ground run.

The book value is exactly that, but the airplane can be flown all the way into the stall if necessary, if I recall, 1.3 Vso is the "max" shortfield speed, not the minimum.

 

[z987k]

The amount of energy you have on touchdown is the amount of energy you have to get rid of in the roll-out. Remember that the energy goes up by the square of your speed - meaning double the speed and you quadruple the energy. Remember also, stall speed varies with weight, power setting, flap setting, ground effect, and that's the "slowest possible touchdown speed" where you're in control. Depending on how short the strip is, and how comfortable you are, an approach speed of around 1.1Vso may be better suited in the last part of the approach. I'd often have two power settings in my head, the one that would give me 1.3 Vso - ish, and the one that would put me right above stall and feel it on the rest of the way from there and transition to this second power setting around 200' AGL. I remember in the 206 coming in with just enough airspeed left to be able to able to flare and feeling the stall buffet an instant before the mains touch - that's the speed to have the shortest possible ground run.

The book value is exactly that, but the airplane can be flown all the way into the stall if necessary, if I recall, 1.3 Vso is the "max" shortfield speed, not the minimum.

Ya, really the horn should be on for the last part of the approach. You should also be as shallow as possible given obstacles. Drag it in low with the the horn on basically.

 

[mshunter]

Ya, really the horn should be on for the last part of the approach. You should also be as shallow as possible given obstacles. Drag it in low with the the horn on basically.

Different than the technique I use/teach. IMO, it should be a power off, or nearly so and steep. Chances are in a short field approach, you are having to fly it in steep because of some type of obsticle. Just a different way to skin it I guess. But yeah, hearing the horn is an important part of the last part of the approach. If it ain't goin off in the round out, you ain't doin it right. *spits chew on the ground*

 

[z987k]

Different than the technique I use/teach. IMO, it should be a power off, or nearly so and steep. Chances are in a short field approach, you are having to fly it in steep because of some type of obsticle. Just a different way to skin it I guess. But yeah, hearing the horn is an important part of the last part of the approach. If it ain't goin off in the round out, you ain't doin it right. *spits chew on the ground*

It depends on the airfield for the angle of the approach. I can show you one's that are going to require a steeper than PTS, and those that you can come in 1 ft above the ground for 3 miles preceding the threshold. However coming in high like there's a 100' tree in the way when there's not means you have a lot more energy than you need.

 

[MidlifeFlyer]

Different than the technique I use/teach. IMO, it should be a power off, or nearly so and steep. Chances are in a short field approach, you are having to fly it in steep because of some type of obsticle.

Not necessarily. Most of my real "short field" landings have been in mountainous terrain and required a steeper approach due to downdraft burbles on final because of lower terrain just before the runway. Steepening the approach without building up airspeed is a very common mountain flying technique.

What I find often helps students with both short and soft field landings is an explanation on how the techniques are useful for things other than "soft" and "short" fields.

But, anyway, stabilized and steep is the PTS version, so the "drag and drop" version, while definitely useful in some situations, is not what is being looked for.

 

[mshunter]

It depends on the airfield for the angle of the approach. I can show you one's that are going to require a steeper than PTS, and those that you can come in 1 ft above the ground for 3 miles preceding the threshold. However coming in high like there's a 100' tree in the way when there's not means you have a lot more energy than you need.

Wow. You mean I've been doing it wrong this whole time!!! I'm lucky I'm not dead! *rolls eyes* I was pointing out a different way to do it. If I had to much energy, how do I still hit my point at a stall?

Screw this part of.the forums. I'm sick of the "there is only one right way" approach to it. I'm punching out.

 

[mshunter]

Not necessarily. Most of my real "short field" landings have been in mountainous terrain and required a steeper approach due to downdraft burbles on final because of lower terrain just before the runway. Steepening the approach without building up airspeed is a very common mountain flying technique.

What I find often helps students with both short and soft field landings is an explanation on how the techniques are useful for things other than "soft" and "short" fields.

But, anyway, stabilized and steep is the PTS version, so the "drag and drop" version, while definitely useful in some situations, is not what is being looked for.

Have a peak at my previous post. I can't multiple quote from my phone.

 

[Blackhawk]

Have a peak at my previous post. I can't multiple quote from my phone.

Wow. You mean I've been doing it wrong this whole time!!! I'm lucky I'm not dead! *rolls eyes* I was pointing out a different way to do it. If I had to much energy, how do I still hit my point at a stall?

Screw this part of.the forums. I'm sick of the "there is only one right way" approach to it. I'm punching out.

I don't think either of them slammed you for your technique- especially Mark who in years of observation on different boards has been one of the most polite posters I have ever encountered.
I have actually used both techniques. I've gone into high mountain strips that were short with nothing around but tricky winds. I've gone into grass strips in NY with 100' trees all around. Several different techniques but I think all share strict airspeed control in common.

 

[shdw]

I don't think either of them slammed you for your technique- especially Mark who in years of observation on different boards has been one of the most polite posters I have ever encountered.
I have actually used both techniques. I've gone into high mountain strips that were short with nothing around but tricky winds. I've gone into grass strips in NY with 100' trees all around. Several different techniques but I think all share strict airspeed control in common.

This.

Obstacles not a problem? Runway 100-200 feet long? You better know how to drag it in.

Obstacles an issue? Runway maybe 400-500 feet long? You better know how to ride the horn the last 100 or so feet w/o power and keep just enough airspeed to stop from kareeming into the ground.

It's not one technique is better. It's which technique will accomplish the goal. As has been said, both need precise control of the airplane. Dragging in with power requires increasing both angle of attack and power gradually during the last 100 feet or so. Where as the steep approach method requires precise judgement of when to flare. Both require on pitch/speed performance.

--

FWIW PTS has no requirement for approach steepness. Only for +10/-5 knots, landing straight, correcting for winds, landing within 200 feet of a point, and stopping in the shortest distance safely possible.

 

[MidlifeFlyer]

I don't think either of them slammed you for your technique- especially Mark who in years of observation on different boards has been one of the most polite posters I have ever encountered.

Well, I've had my moments

But in this case, I was agreeing with him on technique. Just [pointing out that obstacles are not the only reason for a steep approach.