Vref & landing
 

Hello All

Whats is the correct speed for landing a jet ? It has been suggested to me that you should land at Vref !

Im new to a jet and my technique for landing has always been Vref over the threshold and then, it becomes a visual thing.
So in my new type (Hawker 800) I maintain Vref till over the threshold and then when I hear the 20ft call I slowly reduce the power - level off and catch the sink with a bit of nose up which gives a nose up attitude for landing, this way of course the speed bleeds off and on a couple of occasions a fraction of a sec before landing the stall warner has sounded (is this wrong ?), I thought this would be a good thing as it means you are landing at a slow speed and making the most of aerodynamic drag.

Look forward to your replys !!

Spam Up

When I say a fraction of a second, I really mean a fraction !

so tell me then , why is it okay in a C172 and not anything else, im still getting good landings with this technique ?

Spam Up

Vref is supposed to be the min speed at ?50ft' on finals, and I have always been taught that this SHOULD be bled off towards touchdown, and touchdown SHOULD occur at a minimum of Vref-5 - although the shrieks and yells from the other seat stop me doing that!:) Indeed, for the B737, Boeing confirm the above.

Correct (Boeing)technique :

Keep FAS : Vref + X ( for winds <= 10 knots : 5kts) until crossing the threshold, thereafter bleed off the speed during the flare.
For gusty winds the correction "X" to Vref would be 1/2 the steady wind + the gust with a maximum of 20 kts.

Why not VRef crossing the threshold ? to have some margin above the stallspeed.

Ummm. The stall warning in a Hawker is the stick shake!!!
Can I respectfully advise that to land with the stick shaker going is NOT correct. :eek: You are tooooooo slooooow. :=

Vref is the speed at 50' over the threshold. Not the touchdown speed. remember it's at least 1.23 Vs. I wouldn't expect stall warning before touchdown though.

I thought there is a horn also ? whats the horn that sounds then :\?

Definetley no stick shaker going off !




Spam

Hi
 

Opened a good old can of worms here !

been thinking about it ! ( yes it hurt ) could it be the horn for the ventral tank because of a nose high attitude causing fuel to slosh around, but isnt that only at high speed or does it warn you also when the gear is down???

My brain hurts now !

Spam

The horn in a Hawker is really multi-purpose. Much like an attention getter.

From the training manual:

STALL WARNING AND IDENTIFICATION SYSTEM

General

A stall warning and identification system is provided to emphasize the aircraft's natural cues available at the point of stall. The system functions are:

Stick shaker (warning)

Stick pusher (identification)

The system comprises ywo sensing channels, each using an airflow angle sensor vane, a signal summing unit (SSU), and a third sensing channel which uses two underwing pressure-sensing vents and a stall identification sensor.

Warning and identification is provided by two stick shaker motors, and hydraulically operated actuator with two integral electrohydraulic valves respectively, and associated logic, announciators and test switches.

SYSTEM LOGIC

1. It is impossible for a stick push to occur before a stall warning (stick shake)

2. No single active fault of an SSU or relay can cause the operation of a stall valve or the associated red STALL VALVE annunciator.

3. The autopilt is disengaged when a stall warning signal is initiated. This prevents the autopilot from attempting to counteract the resulting stick shake operation or subsequent stick push.

STALL WARNING SYSTEM

The stall warning system uses an electrically driven stick shaker on each control column to provide a physical warning of an approaching stall to the pilots.


There is more but I'm knackered now!

If you are not getting the shaker you are not on the stall warning.
Fuel in the ventral tank may be the reason if you are getting the horn!!!!:ok:

Spam Up. IIRC not all versions of ‘HS 125’ (if any) use the standard margins for calculating approach speed; thus you should follow what is written in the ‘particular’ aircraft’s manuals.

Normally the reference approach speed (Vref) is 1.3 stall speed (Vs), and most aircraft are certificated for a landing at or about 93% of Vref, often rounded to a training recommending to touchdown at Vref-7kts. However, as the 125 originated under UK BCARs many years ago, it claimed grandfather rights to maintain the benefits of reduced landing distance from a better (slower?) approach speed. A different FAA certification was used for the original ‘Hawker’ aircraft, circ 125-400, thus giving another set of figures; notably (to the angst of the Hatfield designers) a ‘lower’ FAR part 23 cert standard.
Later UK build and certified 125s may have compromised slightly in either using revised UK requirements, or moving toward the JAR 25 standard, but, not as I recall, ever having a JAR certification.
Most certifications normally account for an abuse approach and landing from Vref-5 at the threshold with a correspondingly lower touchdown speed – tail strike limit.

In addition to the old regulations being applied to later variants of the 125 (at least the 800?), there may have been other differences between the ‘A’ (USA and FAA cert) and the ‘B’ (rest of the world, UK cert) markets (N.B. there were some FAA certs in ‘the rest of the world’ so check the manuals and who owned the aircraft originally/previously). The AFM should reflect these differences, which might also include Vmo / Mmo.
There may be a reference in the aircraft documentation to the certification standard, i.e. UK BCARs (old), UK – becoming JAR, or FAA. When the basis of certification has been established, you can then check the speed margins for approach and landing from the appropriate regulations;- of course follow the advice in your ‘particular’ aircraft manual might be quicker.

The 125 also has a particular characteristic when landing with full flap, in that the ground effect can feel like sitting on a bubble of air, which cushions the landing. This depends on landing at the ‘correct’ speed and ‘holding off’ at relatively low height (1-2 ft?); if the aircraft is too high or held off too long with speed falling, the bubble can burst resulting in a ‘stiff’ landing. The trick is to ‘burst the bubble’ at touchdown.

That's exactly as i posted !
things haven't changed as much since '65 :)

EXCEPT that we set our bug speed at FAS = Vref + correction ! ( min 5 max 20 kts)

Embaer do a nice little pamphlet called understanding Vref and Approach speeds. I don't have it with me but will print extracts from work tomorrow. Not withstanding the Approach speed additions which for the E-jets are half the head plus all the gust min 5 max 20 this speed is expected to be lead off so you cross the threshold at Vref and touchdown below Vref.

Important to know that Vref is used to ensure landing climb and approach climb gradients so could be considerably higher than 1.3 or 1.23 Vsr.

Cross the threshold...
 

Target speed is usually 1/2 the steady state wind plus all the gust factor (if any). This speed is to be held until crossing the threshold (actually
50 feet). Total additives not to exceed 20 kts...


Example: Vref is 120
Landing Runway 10
Reported surface winds: 060/12G22

So, take half of the 12, which equals 6....and all of the gust factor, which is 10 (22-12)

Vapp = 120 + 6 + 10 = 136

Note: This is a TARGET SPEED....it's not a minimum speed or maximum speed...it's what you try to hold...plus or minus...based on the turbulence and wind speed kicking around.

Speed to reduce to as you're crossing the fence:
120 + 10 = 130

(10 is the gust factor...which you are to hold crossing the threshold, going into the flare)

The idea is to touchdown at a speed not below Vref...or thereabouts....so, you have to be prepared for a loss of the gust factor as you go into the flare...thus, you hold the extra speed (in this case the gust factor of 10 kts). In theory, you could lose 10 kts of wind...lose 10 kts of speed...at the worst moment (in the flare)...that's why you keep the gust factor as you cross the threshold.

In the Airbus, we normally use 'Managed Speed' .....so, these calculations are made for you...a different algorithm.


Fly safe,

PantLoad

Boeing have always avoided recommending the point on final approach at which you deliberately commence to reduce airspeed when the only additive is Vref plus half the steady headwind component.

Example runway 27 and wind 270/30 knots. Half the HW comp is 15 knots therefore approach speed Vref plus 15 knots. Under steady wind conditions the average landing will bleed no more than 3-5 knots off in the flare providing you do not float. . That leaves at least 10 knots excess speed. The free flow airstream (wind gradient) commences around 2000 ft above ground level. Below that, ground friction slows the wind. This suggests in a perfect world you should commence the bleed off of the half the steady HW component at 2000 ft. It is this failure to understand the dynamics of bleeding off the half the steady HW component, that has been a factor in some fast landings and subsequent over-runs particularly on a wet or slippery performance limited runway. Everyone becomes an expert when this point is discussed.

For Airbus VAPP would be only 125 as the correction is limited to the higher of
- 5Kt
- 1/3 HEADWIND (excluding gust) max 15Kt

The 727 didn't like Vref+5 always felt better +10. Other Boeings +5 and the gust loading to a max of +20 is fine. I have never been back to Vref in a jet.

I have never flown the Hawker so I don't know if tailstrike is ever an issue.
Maybe someone with lots of jet experience can give their opinion on whether or not Spam is setting himself up for a tailstrike if things start getting a bit gusty. If it is not an issue in the Hawker it may be in other types that you fly Spam....down the track a bit I mean.

Some observations

(a) The 727 didn't like Vref+5 always felt better +10

If my recollection is correct, the -100 was certificated on the basis of stall speed, while the -200 used minimum steady flight speed, leading to the common practice of carrying a bit extra on the later aircraft. I suspect that most (many ?) of us preferred the -100 for landing ease.

(b) the steady wind/gust (Boeing) rules of thumb reflect the reasonably predictable nature of a steady wind (intentional speed bleed into the flare) and the randomness of gusts (maintain the gust additive). This was discussed at length in an old thread. The slightly different tack adopted by Airbus indicates that the subject is a bit rubbery and that the goal can be achieved by multiple, similar techniques.

(c) the certification landing animal is a flight test black art resulting in a substantial operational fudge factor (100/60 or 1.67) for the final AFM data. Those who have been involved with performance takeoff and landing trials will understand what I mean. Rather than agonise over whether a particular aircraft was certificated to this or that, on the line I prefer to consider the underlying intent of the exercise - which is to stop comfortably on the runway .. and I hold that the AFM is the pilot's friend ..

(i) follow the AFM guidance, fly a stable approach, and you are half way to the hotel

(ii) beware of slow speed approaches unless you have lots of instantaneous thrust to counteract gust problems

(iii) beware of low approach path landings lest the undershoot area beckon you

(iii) beware of high speed or high approach path landings lest the overrun area beckon you

(iii) if the aircraft is somewhere near (say, ± 10 kt) the AFM recommendation at screen/touchdown, and touchdown is in the nominal touchdown region (typically around 1500 - 2000 ft into the runway) then there is a high probability of a successful outcome. Conversely, if not, then perhaps one ought already to be on the missed approach or doing something else appropriate and urgent to address the matter.

Here is the conclusion from Embraers guidance pamphlet
 

SECTION V − CONCLUSIONS
From the certification point of view, landing performance data
presented in AFM/AOM is generated considering that the airplane will
be at VREF once it has reached the threshold and is at a height of 50 ft
above the ground.
From a practical point of view, every pilot knows that almost every
approach is better if the airplane maintains a speed higher than VREF
on final approach in order to assure a speed margin above stall should
turbulent air or variable wind conditions be encountered along the
flight path.
Due to these reasons (or at the Captain’s discretion for safety
reasons), sometimes it is not possible to cross the threshold exactly at
VREF. It must be clear that VAPP must be the upper limit in this case. In
this scenario, the pilot must always keep in mind that the performance
achieved cannot be determined exactly but may be close or better
than the calm wind performance data.
Crossing the threshold with VREF at 50 ft height will always
produce, at least, the predicted performance in the AFM and this is
the actual policy suggested by Embraer.