Why 1.3 Vs for approach?
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"As for the Buffalo accident, I have more than my fair share of knowledge about that one. I rode over to Farnborough on that aircraft, and was a jump seat observer for the show demo flight, two days before the accident. I had a limited background role in the investigation. The reduced energy to flare was an aspect in that accident, but it is my opinion that other factors were large contributing factors too.
16th March 2010 08:01"
I was at DH back then and I heard (from either Bob or Wally at the time) that they thought that Bill the pilot got caught by a bit of reduced performance windshear that ate into what little energy margin he normally had in doing that kind of on-the-edge STOL demo. Was that the crux of it?
OT, but funny how that 6 degrees of separation thing works: I 1990 I was bush flying for an air service in Sudbury and picked up a Globe and Mail travel reporter from a fishing lodge, who turned out to be Linda L, Bill's ex wife. He'd already left to do contract flying back in England. I believe he was killed in that Dash 7 crash in England some years back.
16th March 2010 08:01"
I was at DH back then and I heard (from either Bob or Wally at the time) that they thought that Bill the pilot got caught by a bit of reduced performance windshear that ate into what little energy margin he normally had in doing that kind of on-the-edge STOL demo. Was that the crux of it?
OT, but funny how that 6 degrees of separation thing works: I 1990 I was bush flying for an air service in Sudbury and picked up a Globe and Mail travel reporter from a fishing lodge, who turned out to be Linda L, Bill's ex wife. He'd already left to do contract flying back in England. I believe he was killed in that Dash 7 crash in England some years back.
TIM, we have to be careful with our words here. First, my use of ‘windshear’ would not cover the big windshear events; thus, wind variation during gusts and wind change with altitude might be more appropriate.
Second, your use of ‘adequate’ depends on the context.
For the redefined conditions above, 1.3 Vs should be adequate during the approach and landing in benign conditions (low turbulence), both for large commercial and for GA aircraft. However, without a specific reference and little GA light aviation experience, I don’t know if this is adequate for operations in all conditions.
Commercial aircraft encountering a gust or shear during the flare, might require additional speed to offset engine response, handling qualities (response time), etc. This leads to the debate as to whether speed increments are carried into the flare or not, or that gusts and shears are of less concern during the flare – then there is ground effect, view over the nose, tail strike … etc, etc.
Extract from AC 25-7 Fight Test Guide for Certification. Para 19, re 25.125.
“The term VREF used in this AC means the landing threshold speed (i.e. speed at 50 ft. height) scheduled in the AFM for normal operations. The minimum value of VREF is specified in FAR 25.125(a)(2) as 1.3VS, which provides an adequate margin above the stall speed to allow for likely speed variations during an approach in low turbulence. ”
Second, your use of ‘adequate’ depends on the context.
For the redefined conditions above, 1.3 Vs should be adequate during the approach and landing in benign conditions (low turbulence), both for large commercial and for GA aircraft. However, without a specific reference and little GA light aviation experience, I don’t know if this is adequate for operations in all conditions.
Commercial aircraft encountering a gust or shear during the flare, might require additional speed to offset engine response, handling qualities (response time), etc. This leads to the debate as to whether speed increments are carried into the flare or not, or that gusts and shears are of less concern during the flare – then there is ground effect, view over the nose, tail strike … etc, etc.
Extract from AC 25-7 Fight Test Guide for Certification. Para 19, re 25.125.
“The term VREF used in this AC means the landing threshold speed (i.e. speed at 50 ft. height) scheduled in the AFM for normal operations. The minimum value of VREF is specified in FAR 25.125(a)(2) as 1.3VS, which provides an adequate margin above the stall speed to allow for likely speed variations during an approach in low turbulence. ”
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Dakota, official report into the Buffalo accident can be found here
http://www.aaib.gov.uk/cms_resources...5%20C-GCTC.pdf
http://www.aaib.gov.uk/cms_resources...5%20C-GCTC.pdf
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A bit more "crux" than that...
Additional information of a "personal account/opinion" nature, can be found here:
http://www.alexisparkinn.com/photoga...nd_DHC5D_story[1].pdf
Bill was a fine person of amazing skill and judgement, who was extremely giving, and willing to extend himself for the benefit of others. It was a Dash 7 in which he, and Mic Saunders, came tho their end. If Bill and Mic could not get a Dash 7 safely back on the ground, no one could!
Dakota, perhaps you're thinking of "Helga" L?
But, I'm guilty of thread drift.... Sorry G.
Additional information of a "personal account/opinion" nature, can be found here:
http://www.alexisparkinn.com/photoga...nd_DHC5D_story[1].pdf
Bill was a fine person of amazing skill and judgement, who was extremely giving, and willing to extend himself for the benefit of others. It was a Dash 7 in which he, and Mic Saunders, came tho their end. If Bill and Mic could not get a Dash 7 safely back on the ground, no one could!
Dakota, perhaps you're thinking of "Helga" L?
But, I'm guilty of thread drift.... Sorry G.
Last edited by Pilot DAR; 21st Mar 2010 at 16:49.
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Thanks DAR
Why am I not surprised. Suits, don't you just love them - not.
The deHavilland company position was “pilot error”, and yes the pilot did fly with no margin for a problem – and had a problem. This was done in accordance with the desires of the pilot’s superiors in the company, who, of course, later denied such instruction. When Bill returned to Canada, and was told that his resignation had been accepted. It had not been offered!
Can someone point me in the right direction about energy and the flare and arresting rate of descent?
When I said the first bloke said 92 knots the second chap was horrified saying I could have killed myself (tell me about it!) because 92 knots was an aircraft carrier landing just above the stall and with power on until after touch down. That explained a lot.
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Very likely fell into the category of "engineering judgement" .. as in ... "what do you reckon ? .. how about 1.3 as a rough enough place to start ?"
Same thing with the 70mph SE stall limit ... finger in the wind starting point which became enshrined as Gospel.
Similar thing with the 50ft screen height (although that was based, apparently, on tree heights around a particular military parade ground during a demo in the olden days) ...
Same thing with the 70mph SE stall limit ... finger in the wind starting point which became enshrined as Gospel.
Similar thing with the 50ft screen height (although that was based, apparently, on tree heights around a particular military parade ground during a demo in the olden days) ...
Mr. Fox
I would believe so because it would still have to be built to the parameters of the established codices i.e it still have to meet the landing distance requirements or have at least special procedure approved as equilvants
PS I love this thread! very enlightening for someone who really tries to think what were the originators thinking? to me history is so important because you can't know where you are going if you don't know from whence you came
PA
I would believe so because it would still have to be built to the parameters of the established codices i.e it still have to meet the landing distance requirements or have at least special procedure approved as equilvants
PS I love this thread! very enlightening for someone who really tries to think what were the originators thinking? to me history is so important because you can't know where you are going if you don't know from whence you came
PA
Do a Hover - it avoids G
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Mr Fox
When the certification of 'Concorde to be' was being discussed back in 1964 to 1965 time at Bedford (with the ARB in those days) it was decided to use the zero rate of climb speed because the slender delta did not stall in the conventional way but at VZRC you can only accelerate by lowering the nose and reducing drag thus giving away some height.
To the pilot such a procedure had obvious comparisons to the traditional stall.
JF
When the certification of 'Concorde to be' was being discussed back in 1964 to 1965 time at Bedford (with the ARB in those days) it was decided to use the zero rate of climb speed because the slender delta did not stall in the conventional way but at VZRC you can only accelerate by lowering the nose and reducing drag thus giving away some height.
To the pilot such a procedure had obvious comparisons to the traditional stall.
JF
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it was decided to use the zero rate of climb speed
But I presume that demonstration requires the use of considerable power, as opposed to none. A 1.3Vs approach could obvioulsy be accomplished at zero thrust (all be it rather steep). Vzrc could not be demontrated power off, unless it were being flown at a speed determined during previous partial power flying?
So, instead of 1.3Vs, approach becomes 1.3Vzrc, power off, if that is the circumstance?
Do a Hover - it avoids G
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Sorry DAR
I should have explained more fully. Vzrc is measured at full thrust in level flight. As you slow down below Vmd you have to add thrust until at some very high alpha - 35 ish - you finish up at full throttle and unable to climb or turn. This is Vzrc. The recovery from this is similar to a stall in that you dump alpha (and so drag), sacrifice a small amount of height, accelerate and fly away.
The snag with a multi-engined slender delta is that Vzrc is clearly thrust dependant. This is unlike Vs where there is only a small increase of you loose a donk. So one needs to consider Vzrc measured with one engine failed in order to handle the typical certification requirements of being able to continue to climb following an engine failure.
What margin (like the 1.3) you need to add to a 3 engined Vzrc would probably be type dependant. I don't know what was eventually chosen for Concord cert as I left Bedford to go hovering in 1967.
Hope this helps.
JF
I should have explained more fully. Vzrc is measured at full thrust in level flight. As you slow down below Vmd you have to add thrust until at some very high alpha - 35 ish - you finish up at full throttle and unable to climb or turn. This is Vzrc. The recovery from this is similar to a stall in that you dump alpha (and so drag), sacrifice a small amount of height, accelerate and fly away.
The snag with a multi-engined slender delta is that Vzrc is clearly thrust dependant. This is unlike Vs where there is only a small increase of you loose a donk. So one needs to consider Vzrc measured with one engine failed in order to handle the typical certification requirements of being able to continue to climb following an engine failure.
What margin (like the 1.3) you need to add to a 3 engined Vzrc would probably be type dependant. I don't know what was eventually chosen for Concord cert as I left Bedford to go hovering in 1967.
Hope this helps.
JF
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I always thought 1.3 was arrived at as a compromise value that provide good stall and energy margins in the approach configuration, while also being close to max LD in the power off glide for most a/c.
If you can find a copy of the ICAO "Final Report of the Standing Committee on Performance" (cira 1958) you will probably find an explanation. Which would include a lot of probabilty analysis on things like the probability of being at a lower speed than desired for all sorts of reasons. It was a very interesting read on how pretty well all performance related aspects of transport aircraft developed. Of course it didn't include more recent changes such as performance based on Vs 1g and minimum level flight speed (Concorde).
Don't forget that most certification standards require a safe landing from Vref minus some thing - usually 5 knots. I have seen a few light aircraft which cannot comply (at 1.3vso minus x), so Vref ends up being more than 1.3Vso. There may well be transport aircraft with the same situation.
Pity I dumped my ICAO report along with a bunch of AAEE documents during a recent "life decluttering" event!
Don't forget that most certification standards require a safe landing from Vref minus some thing - usually 5 knots. I have seen a few light aircraft which cannot comply (at 1.3vso minus x), so Vref ends up being more than 1.3Vso. There may well be transport aircraft with the same situation.
Pity I dumped my ICAO report along with a bunch of AAEE documents during a recent "life decluttering" event!
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I did a little asking around the office and found that 1.3 Vso as a single standard value is no longer used, in transport catagory airplanes at least. The value is variable can be down to the low 1.2s (the CRJ900's Vref is 1.23 Vso) and is a function of various factors that takes into account stall behaviour, the spread between Vref and V2ga etc etc. Basically Vref is set to be as slow as possible while having sufficient energy for stall margin, arresting sink at the flare, and the transition from Vref to climb at V2 - V2+10 for the go-around case.
Dak435, I am pretty sure that you will find that an approach speed factor of 1.23 is acceptable when applied to Vsr ie 1g stall speed. The factor of 1.3 was applied to stall speeds measured as the minimum reached during the stall manoeuvre. The change from minimum speed in the stall to Vs 1g started around the time of the A320 development.
While FAA special conditions were applied for a number of years it seems that the issue was formalized after the issue of the NPRM linked below.
1-g Stall Speed as the Basis for Compliance With Part 25 of the Federal
Essentially 1.3 Vs0 and 1.23 Vsr are about the same speed.
While FAA special conditions were applied for a number of years it seems that the issue was formalized after the issue of the NPRM linked below.
1-g Stall Speed as the Basis for Compliance With Part 25 of the Federal
Essentially 1.3 Vs0 and 1.23 Vsr are about the same speed.
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*Warning* PPL *Warning*
Help.
If Vso is the stall speed str and level in the relevant landing configuration isnt this a 1G condition ? (I assume the test pilot isn't attempting a flick roll entry!!)
Why is Vr a better or correct 1G stall?
Yours confused (as always) TIM
Help.
If Vso is the stall speed str and level in the relevant landing configuration isnt this a 1G condition ? (I assume the test pilot isn't attempting a flick roll entry!!)
Why is Vr a better or correct 1G stall?
Yours confused (as always) TIM
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And also out of interest how frequently have people in the business seen a sentence like this in a document
"has proven adequate; and harmonize the applicable regulations with those proposed for the European Joint Aviation Requirements-25 "
(Just joking honest) TIM
"has proven adequate; and harmonize the applicable regulations with those proposed for the European Joint Aviation Requirements-25 "
(Just joking honest) TIM
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The new methodology requires correction for the load factor being below 1'g' in a typical demonstration. So Vsr (or Vs1g as it was once known) will be about 5% higher than Vso (or Vsmin, to give it one of its older names), for the same aircraft and similar manoeuvre.
So Vref based on Vsr is 1.23*Vsr, and if you substitute that "about 5%" factor you get 1.23*1.05*Vso which is about 1.30Vso (technically it's 1.2915, but I did say "about") - just like zzuf says, about the same value.