Why 1.3 Vs for approach?

[RansS9]

Why 1.3 not 1.2/1.5/1.1 etc?

Is this a value arrived at by experience or is there some objective reason why 1.3 was chosen?
Is this valid across most aircraft types and if not is there a reason why not?
Does this include an element of gust protection or does that have to be added separately?
(I'm particularly interested as to how it effects the light end of the aviation spectrum.)

Thanks TIM

[woodja51]

1.3 VS

Hi,

not a test pilot so happy to be corrected but my understanding of how 1.3 Vs is arrived at might be the following.

At the 3 degree glide path that most jets fly at ( based around maximum rate of descent that the gear can handle before overstress ( which I think is up to 640 fpm at max landing weight and 300 fpm at max to weight so I believe) there exists a speed that the aircraft rate of descent can be arrested in the flare. ( I even think there is a G rate for that somewhere) such that the airspeed bleeds off to achieve a speed of touch down just higher than the actual stall speed of the aircraft.

Stalling 250 tonnes of aluminium from even a few feet above a runway would be quite a shock to the jet.

So that is basically how they arrived at 1.3 - guess it works empirically.

Of note is that the Airbus family of FBW jets due to the protection measures in the flight controls are actually certified to slightly lower Vapp speeds if my memory serves me correctly.

Might be totally bogus answer but that is what I am lead to believe.

13000 hours total on both 777 and 330s 707s and stuff like that... but not an B Aero Eng.!

[sycamore]

It is in reality 1.3Vso,which is the stalling speed in the landing configuration. 1.3Vs relates to a clean configuration,which will be higher for those aircraft which have flaps,retractable u/c,slats etc.

[RansS9]

1.3Vso my mistake.

I didn't know the 3 degree slope was in part chosen because of maingear limiting rates of descent in jets. However in my defense I don't know quite alot of things and it seems to get worse as I get older!

It appears that the 1.3 seems to be Large Jet-centric (no problem there) but does that make it as valid in the light aviation sector ?

TIM

(13,000hrs 777, 330's 707's cf 200hrs C152, Robin160, Pitts S1
Okay you definately win the TOP TRUMPS )

[Brian Abraham]

Some of the relevant FARs if it helps.

§ 25.125 Landing.

(a) The horizontal distance necessary to land and to come to a complete stop (or to a speed of approximately 3 knots for water landings) from a point 50 feet above the landing surface must be determined (for standard temperatures, at each weight, altitude, and wind within the operational limits established by the applicant for the airplane):

(1) In non-icing conditions; and

(2) In icing conditions with the landing ice accretion defined in appendix C if VREF for icing conditions exceeds VREF for non-icing conditions by more than 5 knots CAS at the maximum landing weight.

(b) In determining the distance in paragraph (a) of this section:

(1) The airplane must be in the landing configuration.

(2) A stabilized approach, with a calibrated airspeed of not less than VREF, must be maintained down to the 50-foot height.

(i) In non-icing conditions, VREF may not be less than:

(A) 1.23 VSR0;

(B) VMCL established under §25.149(f); and

(C) A speed that provides the maneuvering capability specified in §25.143(h).

(ii) In icing conditions, VREF may not be less than:

(A) The speed determined in paragraph (b)(2)(i) of this section;

(B) 1.23 VSR0with the landing ice accretion defined in appendix C if that speed exceeds VREF for non-icing conditions by more than 5 knots CAS; and

(C) A speed that provides the maneuvering capability specified in §25.143(h) with the landing ice accretion defined in appendix C.

(3) Changes in configuration, power or thrust, and speed, must be made in accordance with the established procedures for service operation.

(4) The landing must be made without excessive vertical acceleration, tendency to bounce, nose over, ground loop, porpoise, or water loop.

(5) The landings may not require exceptional piloting skill or alertness.

§ 25.473 Landing load conditions and assumptions.

(a) For the landing conditions specified in §25.479 to §25.485 the airplane is assumed to contact the ground—

(1) In the attitudes defined in §25.479 and §25.481;

(2) With a limit descent velocity of 10 fps at the design landing weight (the maximum weight for landing conditions at maximum descent velocity); and

(3) With a limit descent velocity of 6 fps at the design take-off weight (the maximum weight for landing conditions at a reduced descent velocity).

§ 25.479 Level landing conditions.

(a) In the level attitude, the airplane is assumed to contact the ground at forward velocity components, ranging from VL1to 1.25 VL2parallel to the ground under the conditions prescribed in §25.473 with—

(1) VL1equal to VS0(TAS) at the appropriate landing weight and in standard sea level conditions; and

(2) VL2equal to VS0(TAS) at the appropriate landing weight and altitudes in a hot day temperature of 41 degrees F. above standard.

(3) The effects of increased contact speed must be investigated if approval of downwind landings exceeding 10 knots is requested.

§ 25.481 Tail-down landing conditions.

(a) In the tail-down attitude, the airplane is assumed to contact the ground at forward velocity components, ranging from VL1to VL2parallel to the ground under the conditions prescribed in §25.473 with—

(1) V L1equal to V S0(TAS) at the appropriate landing weight and in standard sea level conditions; and

(2) V L2equal to V S0(TAS) at the appropriate landing weight and altitudes in a hot day temperature of 41 degrees F. above standard.

A reposting from Vref=1.3Vs? [Archive] - PPRuNe Forums and is the answer given by a performance engineer.

Usually the 1.2Vs and 1.3Vso figures will be specified in the flight manual as it is important for the manufacturer to obtain best field length performance data to help sell the aircraft - the slower the aircraft the better for this consideration.

For some aircraft, depending on the certification basis, the stall speed may have been replaced by the minimum steady flight speed - which may be slightly different to the stall speed - no prizes for guessing in which direction. I believe, for example, that this was the case for the B727-200 and was cited by some as being part of the reason for the difficulty that some pilots (i.e. most of us) had in flaring that model to a nice landing unless a few knots extra were carried.

Also, there may be the odd aircraft where the AFM figures have an increased buffer for other reasons - one has to keep in mind that there are several certification requirements which have to be met simultaneously and it sometimes can lead to unhelpful confusion if one specific requirement is looked at in isolation. For example, the Vmca handling requirement on multis might well cause this to occur.

Another thing to keep in mind is that the manufacturers consider their data to be confidential. As a result, it is usually difficult to find out all (much ?) of the story relating to any SPECIFIC aircraft certification program. Occasionally, the manufacturer will sensibly engage in some horse-trading with the certification authority and this sometimes results in apparent anomalies. Also the use of the grandfather certification basis often leads to the situation where a pilot looks up the CURRENT design standard (e.g. FAR 23 at amendment so and so) - which has not a lot to do with the aircraft in question which was designed to an earlier standard - and then has a hard time rationalising apparent discrepancies.

It is usual to find that the manufacturer is quite considered in his development of AFM and other manuals - one always has a mind to the potential for litigation when including more data than the minimum required by the certification authorities. This then, quite naturally, encourages pilots to engage in speculation.

(b) One has to be careful that calculations to work out buffers over stall are done in CAS - strictly this should be EAS but the difference is not a concern for stall speeds. If done in IAS then the PEC (position error correction for static source errors) can make for strange results.

I think that we all have seen the small Cessnas deeply in the stall with the ASI indicating VERY low figures - which means only that the position errors have grown dramatically when the aircraft is well within the stall - and has little to do with the aircraft's actual speed.

One has to keep in mind at ALL times that the ASI is measuring pressures, NOT (and NEVER) speeds. The pressure reading's being scaled to speed is based on quite limited and limiting assumptions - if these assumptions don't apply for some reason, then the ASI reading is total high quality garbage - how many jets have had static obstructions and then been flown into the stall while the pilots have erroneously believed the aircraft to be overspeeding ? The same logic applies to the need to calculate TAS - the basic ASI doesn't give TAS to the pilot. It is the old airmanship consideration - get the whole story (or as much of it as might be reasonably available) before over-reacting to something which looks or feels strange or rather out of the normal experience.

It is worth noting that the flight test determination of PEC near the stall takes a considerable proportion of the certification flight testing program.

For light aicraft.

§ 23.73 Reference landing approach speed.

(a) For normal, utility, and acrobatic category reciprocating engine-powered airplanes of 6,000 pounds or less maximum weight, the reference landing approach speed, VREF, must not be less than the greater of VMC, determined in §23.149(b) with the wing flaps in the most extended takeoff position, and 1.3 VSO.

(b) For normal, utility, and acrobatic category reciprocating engine-powered airplanes of more than 6,000 pounds maximum weight, and turbine engine-powered airplanes in the normal, utility, and acrobatic category, the reference landing approach speed, VREF, must not be less than the greater of VMC, determined in §23.149(c), and 1.3 VSO.

(c) For commuter category airplanes, the reference landing approach speed, VREF, must not be less than the greater of 1.05 VMC, determined in §23.149(c), and 1.3 VSO.

[Oktas8]

In my limited experience many of the Regulations and FARs are based on what people thought was sensible back in the day when men wore leather and rules were few.

Could it be that 1.3 Vso was not originally "chosen" by anyone in particular, but was used in the days of biplanes as a reasonable speed at which to cross the fence? Any slower and you'd not have energy left to flare, any faster and you'd be too fast to achieve the desired three-point landing attitude in a reasonable distance.

O8

[Shawn Coyle]

I'm particularly interested in the 'energy left to flare' statement - where does one find info on how that energy is calculated?
The reason for the interest is that there was an accident to a Dehavilland Buffalo at Farnborough many years ago when a short landing was being demonstrated at a very slow airspeed. Flare didn't do anything and a very hard landing ensued.
Can someone point me in the right direction about energy and the flare and arresting rate of descent?

[Deltabravowhiskey]

1.3 is more than arbitrary, it insures that sufficient energy (speed) is retained during normal speed deviations in normal conditions. In turbulent conditions you can go rapidly from stabilized approach to stick shaker or full stall instantly, in which case an approach using 1.5 may be called for.

Conversely, in smooth air you can fly a 1.1 approach to minimize the runway used.

Rather than focus on speeds you would need to look at this first from the perspective of AOA (Angle of Attack). In quite a few aircraft speeds aren't even used and AOA is the primary speed indicator for approach, landing and other phases of flight. This allows the aircraft to be flown with sufficient stall margin at any weight in any condition (Flaps up, down, engine out...pick one), however this does not provide you with any way of knowing (without calculating weight and speed) how much runway will be used for the landing.

In larger aircraft the stick shaker/stall sensor are in fact getting inputs from an AOA probe. AOA is the most accurate way to instantly check where you are flying in respect to the wings performance.

The use of speed allows operators to calculate (based on projected weight) predicted performance in respect to the amount of runway required for landing. Weight will be based on performance generated during flight testing from many factors including AOA. This for legal reasons is perhaps the easiest method to insure suitable runways with sufficient runway length are selected for the conditions.

[Pilot DAR]

I equate the "energy left to flare" concept fairly closely between fixed and rotor wing aircraft. Though I have not attempted it, I would imagine that a stabilized autorotation could be achieved at less than the minimum specified low rotor RPM. When you got to the ground, there would be little or no reserve energy in the rotor to be available to create upward acelleration of the helicopter to reduce descent rate. Landing gear testing....

I do have considerable experience with the differences between normal, and STOL kid equipped aircraft. The STOL kitted aircraft may demonstrate a reduced power off stall speed. This could lull the pilot into a false confidence that a reduced glide speed is safe. The lower glide speed is possible, but the pilot has now surrendered the extra energy reserved in the aircraft, which will be used up to change the direction of the aircraft from somewhat downward, to hardly downward, along the runway. Thus, when the pilot pulls to flare, the aircraft cannot accelerate upward (which would simply be a reduction in downward speed), and just impacts the runway at the extablished descent rate.

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.

[1.3VStall]

Anything other than 1.3 just wouldn't do!!!

[RansS9]

I wonder if anyone can shed light on the historical aspect i.e. at what point in the History of Flight / Aerodynamics/ Aviation Regulation did 1.3 start appearing? It might shed some light on why 1.3 was chosen and if it was to be applied to a specific aircraft group or to aircraft in general.

Assuming ,as sounds most sensible , it relates to energy reserves available for the flare does that mean it is valid across aircraft groups (i.e everything is more or less proportionate and holds true whether 747 or 152) or is it primarily relevant to large aircraft only in which case what values should be used for the other groups?

TIM

[Oktas8]

1.3 Vs0 certainly applies to light aircraft as well as large airliners, although airliners use more complicated concepts as well, like 1.23Vsr0 for example.

As an aside, many light aircraft also need to fly at around 1.2Vs1 after take-off from a short runway. It's not specified in rules, but it does work. There are all sorts of relationships between speeds in most aircraft that arise simply because of common design characteristics across the worldwide fleet - engine power-to-weight ratio, wing aspect ratio, stability requirements and so on.

[Daniel_11000]

1.3 Vso is to take into consideration not only for the approach phase of the landing, but also for the go-around – just imagine flying at, say 1,1 Vso and then try to go-around – on most low-powered aircraft you will probably mush down to the runway, and this may also occur on some jet aircraft flying on the reverse side of the speed-power curve.
For what refers to historical development of the concept, I am pretty sure that the 1.3 figure follows from years of experience, analysis, certification flying testing, millions of flight hours , and – unfortunately, thousands of accidents.

[IGh]

Certification req' changed from '38 to 1941

Why 1.3Vso?

For questions relating to such epistemology of Aircraft Performance guidelines in certification, generally one can go to Joop Wagenmakers’ book, and papers.

But, for the initial appearance of the specific “1.3Vso” in Airworthiness Regulations, this wording predates the Turbojet revolution and predates swept wing aircraft. This specific “1.3Vso” was included the CAR 4b, also included in the earlier CAR 4a (1950), and during the 1940’s while using CAR Part 4.

Before that, certification was conducted to earlier standards defined by CAR Part 4, dated May 1938. Nothing about “1.3 Vso” appears in the 1938 wording, but you can see two mentions of speeds, one using a lower limit of “20 per cent in excess of stalling speed”, and mentioning a second high limit of “ at a speed not in excess of 140 per cent of the maximum permissible landing speed ” [see paragraph 04.704 from 1938 describing “balance”].

CAR Part 04 (1938), “Airplane Airworthiness”, dated May 31st, 1938

paragraph 04.700(b) defines the max landing speed (70 mph);

Paragraph 04.71 “Modified performance requirements for airline carriers”.

04.713 "In no case shall the provisional weight exceed a value corresponding to a landing speed of 5 miles per hour in excess of that specified in [paragraph] 4.700, a take-off distance of 1,500 feet in the case of landplanes, or a take-off time of 60 seconds in the case of seaplanes…."

In the 1938 version of CAR 4, the SPEED to derive Landing DISTANCE was complicated:

04.730 Flare and landing. Under the most critical center of gravity ... fully throttled, propellers ... low pitch and with high lift devices ... the horizontal distance to come to a full stop ... from the start of a normal flare at 50, 100, and 150 ft ... the optimum speed corresponding to each flare ...

CAR Part 4, “Airplane Airworthiness”, Change dated April 1st, 1941, first offers alternate wording relating to Vso, but the wording differs slightly:
Paragraph 04.7503

“Landing. The horizontal distance required to land and come to a complete stop from a point at a height 50 feet above the landing surface, subject to the following conditions:
(a) … steady gliding approach shall be maintained with a true indicated flight path airspeed not less than 130 percent of stalling speed …” [that from Apr’41].

The change in wording (to include “1.3 Vso”) occurs with the next change in Nov’ 1943, see paragraph 04.7533-T(a) which specifically cites that “1.3 Vso” speed at 50’ for use in determining Land Distance.

To understand the “why” of the change, consider this time period just prior to the onset of the “1.3Vso” in CAR Part 4, & consider flight test projects of 1938 and 1939. At Boeing, these projects were the B314 and then the B307 Stratoliner .

FF of B314 (flying boat) was on 8Jun38; Eddie Allen, and Earl Ferguson as pilots and Bill Lundguiest acting as F/E. Designed for Pan Am, which was already operating Martin M-130 "China Clippers" and Sikorsky S-40 and S-42 seaplanes.

Boeing Model 307 "Stratoliner" first flight on 31Dec38. TWA started service on 8Jul40 with a 14:09 time from LGA to Burbank, with stops.

Perhaps those 1938 Regulations for certification were too limiting to advance beyond the B307, because of the fixed maximum “distance” for Takeoff, and fixed limit “speed” to derive landing distance, and a climb time to 300 feet (but not a gradient nor distance, only TIME to height).

From B307 flight test notes you can see some examples of performance, the first two tests were taxi-testing, then FF:

First Flight away from a runway was done on TEST # THREE of 31Dec38: Crew aboard: Allen, Ferguson, West, Barr, Gaylord. T/O Gwt = 32,000 Lbs. Planned test for directional stability on ground. Time from brake release to airborne was 9 seconds, flight then continued due to limited runway remaining. Attempted to retract Landing Gear, but the Tail Wheel failed to retract. Pilot's comments from his report, "Good rolling moment due to yaw."

Test # FOUR done 4Jan39: Taxi testing focused on brake pressures and temperature. Flt time = 1hr 48 minutes; logged engine time as 3 hrs. Crew was Allen, Barr, Ferguson, West, Cram, Jewett, Anderson. At T/O Gwt of 37,000 Lbs, T/O distance was 800 feet into a light SSE wind of 3 mph; after tracking 2300 feet along and above runway the height was 100 feet, using 1100 hp…. [flight test reports are in Boeing Archives].

Mainly, it seems that other reg's of 1938 boxed-in any further development; and so the multiples (130% &ct) of a new ship's Stall Speed helped get newer/faster aircraft certified, even if the new ships flew approach faster than the 75 mph certification-limit of 1938.
= = = = = = = =

The official explanation for the CAR [Performance Section] revision of 1941 appears in CAM 04T, three years after the revised CAR (and the onset of war).

CAM 04, Nov’41, the “Introductory Note” , flatly tells the reader to wait for a future explanation about these 1941 new changes to CAR’s Performance section:

“this manual contains material which is intended to interpret and explain the airplane airworthiness requirements specified in Part 04 of the Civil Air Regulations … This edition of CAM 04 contains material pertaining to CAR 04.0 through 04.4. The remaining sections of CAR 04 will be covered by future additions….

In the 1944 Manual, the explanation appears, in various paragraphs.

http://ntl1.specialcollection.net/sc...&site=dot_cams

CIVIL AERONAUTICS MANUAL 04-T,
TRANSPORT CATEGORY REQUIREMENTS, NOVEMBER 1, 1944

04.753-T - Required Performance and Performance Determination

04.7530-T - Stalling Speed Requirements

The limitations which are imposed by the requirements of this section upon the stalling speeds have been dictated primarily by the effect of the speeds at which it is necessary that the airplane be operated during an approach and landing under adverse weather conditions, upon the safety of that operation. All of the airplanes which had been used in civil operation had, at the time these regulations were written, been designed to comply with a maximum landing speed requirement which had in no case exceeded 70 MPH where passengers were to be carried in the airplane. ... it was considered unwise to abandon ... because the alternative limitations are not absolute ... therefore, possible by the installation of a sufficient amount of power ... and by ... runways long enough, to design an airplane having stalling speeds far in excess of those with which we have been familiar.

04.7533-T - Landing Determination

"... minimum approach speed of 1.3 Vso ... is intended to provide a reasonable margin above the stalling speed."

[RansS9]

Very interesting, Many thanks TIM

[Oktas8]

just imagine flying at, say 1,1 Vso and then try to go-around – on most low-powered aircraft you will probably mush down to the runway

No. Most aircraft most of the time can perform a go around at any time on the approach up until touch down, even in the flare (when you might be at or below the S&L stalling speed).

Exceptions occur, like very hot or high conditions. They're - umm - exceptional.

[Daniel_11000]

Oktas

Oktas,
there are specific performance/ HQ requirements also for Part 23 a/c which need to be satisfied also during the go-around phase, i.e transition characteristics from approac -to-go around phase, approach-climb performance, etc.
If the approach is flown at 1.1Vso , in my opinion most Part 23 aircraft will not be able to meet those requirements (either performance
and/or HQ)
Daniel

[IGh]

Tail damages -- Low ENERGY attempts at Balked Landing

Mentioned two slots above:

"... aircraft ... can perform a go around at any time on the approach up until touch down, even in the flare ... below the S&L stalling speed ...."

Gees, that old pilot perception again:
Under those conditions above, pilot could pull backstick, get ANU pitching moment, impact -- but any G/A is an un-demonstrated maneuver. See long discussion, and documents, and mishaps, cited in slot #50, #55, #61, #73 in thread:
http://www.pprune.org/tech-log/35069...ar#post4544029

[safetypee]

Shawn, Re #7 ‘energy’. A long time ago, I had several informal meetings with Dr Pinsker (RAE Bedford) to discuss landing. One aspect, which I think I understood from his expert views (Aero D and landing), was that the landing maneuver is about the distribution of energy.

In the short term, wing generated lift can be considered as energy, this energy is ‘distributed’ by the elevator.
Important parameters, apart from the total energy, are the rate of distribution and rate of loss of energy; both related to Cl / alpha and elevator power (the ability to access the energy).
For a conventional aircraft, the approach speed must provide sufficient energy to counter the expected energy loss during the landing flare. Thus, the flare time is proportional to Cl / alpha, which in most aircraft is about 7 sec. The duration of the maneuver is sufficient for reasonable iterations in the closed loop control.

STOL aircraft probably have greater capability to access the energy quickly (higher Cl / alpha), thus, they should be able to flare later – shorter flare time. However, this may be at the expense of landing finesse as there may be less iteration judging the flight path. There may even be a point at which the landing becomes open loop; the nearest to this might be landing performance testing.

During a steep approach, a conventional aircraft would have to flare earlier (longer flare), but in a timescale to achieve a shallow flight path before the energy available is used up; if not then a higher approach speed would be required (more energy). A STOL aircraft may be able to use the shorter flare time as the wing characteristics and shorter flare time enable a flight path change with lower loss of energy, but again there is a control loop limit.

There are deviations from this, e.g. blown flap effects (C17) and direct lift control. The latter was demonstrated by RAE (BAC 1-11) where a conventional flare from 6 deg required 100ft, but wing spoiler DLC enabled 35 ft. The tradeoff was a higher approach speed due to the spoiler-out reduced stall margin (not a need for energy), which enabled a very precise touchdown, but unfortunately without commercial advantage of a reduced landing length. STOL aircraft generally need a STOL wing.

Re 1.3 VS, this appears to be an empirical value which provides capability for landing or GA, and to counter gusts and windshear during the approach. The energy for landing is probably met at Vref – 5 kts as indicated by certification requirements. However, I suspect that -5 kts is an approximation for a 5% safety margin, thus a safe min approach speed at 3 deg could be 1.25Vs.
A GA during the flare (as energy is reducing) will require additional energy, which in this case must come from thrust. The maneuver is quite safe if considered as being more like a landing touchdown (which might occur) and subsequent take off. A hazard is touching down before the runway (which is where I believe #18 comes from).
The skill is to is to achieve level, flight, accelerate, and then clean up before attempting to climb. Sufficient thrust will available in 8 sec as required by certification.

[RansS9]

"Re 1.3 VS, this appears to be an empirical value which provides capability for landing or GA, and to counter gusts and windshear during the approach. "
So, with respect to GA light aviation, 1.3Vso includes both adequate energy reserves for the flare manoeuvre PLUS safety margins for gust / windshear ?

TIM