Approach Speed Additives

[RansS9]

(Not sure if this is right place to post, if moderator has better idea no problem)

In calculating the Approach speed for a DC9 additives toVref included;

---half the steady state wind

---half the gust factor (assume gust factor is differencebetween steady state and max gust)

With minimum additive +5 and maximum additive +20.

I understand (or at least think I do) the reason for thegust factor but don’t understand thesteady state wind factor. Is this some sort of low level wind shear protection?And are these figures scalar or suitably resolved vector wind speeds ?

Thanks TIM

(If you hadn’t already worked out I don’t push heavy ironaround for a living just low hours PPl with an interest.)

[safetypee]

TIM, a widely accepted view (folklore ?) of the steady wind additive is to accommodate wind-shear (decrease) due to altitude gradient. However, there may be wide variations in this (as with gust factors) amongst manufacturers and operators depending on aircraft type and operational situations.
The headwind component is normally considered, but the side-force component, especially of gusts should not be discounted when considering maximum crosswind recommendations (manufacturer/operators).
I would add that it is very important to adjust the landing distance required for any speed additive which might be retained until the threshold, and also consider that any expected decrease in the additive might not occur.

[Genghis the Engineer]

I agree with you SafetyPee. This scuttlebut about adding margins to Vref for gusts has been around since Pontious was a pilot, but so far as I know is unsupported by any formal evaluation or research.

My experience is that similar margins have value in small aeroplanes with a high drag / low inertia profile. However I don't believe that this is particularly sensible practice in the majority of cases.

Rans -can you give a reference for these factors you're using - I have a suspicion you may be looking at something derived from barside banter, rather than certified operating data.

G

[RansS9]

Thanks for the replies.
Unfortunately I don't have certified air data nor the actual aircraft's POH or what ever the equivalent is for heavy metal.
Just out of interest I dug out my copy of"Handling the Big Jets" by D.P.Davies. He suggests steady state (presumably that component along the runway although not clarified) upto 15kts as protection against windshear; with half the gust component upto max 5kt additive. Total additives not to exceed 20+ unless "exceptional conditions". Not sure if Mr. Davies qualify's as barside banter.
Surely with all the flight tesing that is done; all of the accumulated low level weather data; the cost of building runways; the critical flight phase; and the seriousness in under/overestimating approach speed that aeronautical engineers have known the answer to the question "What Approach speed?" for years!

TIM

[BOAC]

barside banter - since Pontius was a Pilot (and in my decades of barside banter) has always been half the steady headwind component and all the gust increment to a max of Vref+20kts. Boeing certainly drink at this bar.

Of course, for those foolhardy folk who trust the automatics, set Vref+5 and rely on the autothrottle. to sort out the gusts.:eek:

[RansS9]

Oooops my mistake the initial should have read "All the gust factor" upto max total additive+20.
Seems McDonnell Douglas and Boeing must have been drinking partners, perhaps that's why they merged.

[BOAC]

Just 'barside banter' Rans:)

[safetypee]

“Speed additives …they have always been like that…”
Yes, but when compared with the margins applied to modern commercial aircraft then I wonder if further justifying explanation is now required.
Landing performance is based on Vref, which requires satisfactory handling characteristics (not necessarily ‘easy’, but safe and with acceptable workload). Also there is a demonstrated landing margin of Vref-5 at the threshold, without power adjustment, and without a tail strike.
For an approach at Vref there is a 20% margin to stall warning and a further 10% to the actual stall; this represents approximately 20/30 kts for most aircraft.
Most manufacturers have a small addition to Vref for the approach which accommodates errors in flight accuracy (flight technical error).
The resultant approach speed Vref+5, +/- 5 (basic bug speed) is therefore a target approach-and-landing speed. However, many operators use this as a minimum speed and thus build in a further speed addition, which may not be necessary.
Any wind additions increase the basic bug speed, and if this is also used as minimum, vice a target speed, it will provide a more than sufficient margin during an approach.

What are the most likely windspeed shears and gust extremes?
Are modern airframe designs more able to accommodate these; are modern engine response times / auto throttles more able to maintain speed?
Are operators increasing approach speed more that could be justified and thus this bias is contributing to an apparent increase of overrun incidents?

[BOAC]

I understand the performance figures used are 'good' up to Vref+20 hence the 'limit' for additives.

[john_tullamarine]

+20 harks back to BCAR considerations and, perhaps, further than that - one would have to ask a really old dinosaur ..

This thread (http://www.pprune.org/tech-log/10456-737-wind-increment-vref.html) discussed the subject at some length.

[rigpiggy]

last year I had an approach 20 deg off CL, 38 gust 61. needless to say we added a few knots over 20

[flarepilot]

rans

glad you corrected yourself

it really isn't that hard...always add five knots to vref

do what you said up to twenty knots


if you are in a situation where the numbers work out to adding more than twenty knots, consider going to another airport.

HOWEVER

the DC9 is quite able to tolerate higher speeds...I've seen vref plus 50 with flaps 50 (which are no longer allowed for noise abate) but the runway was 10,000 long and the winds were a bitch plus.

normally touchdown at vref to vref minus five.

I encourage you to read a book called, "fly the wing" by webb...a former DC9 checkairman for eastern and an acquaintance (RIP).

he has a great deal to say about wings and wind...and even use of a higher Vr for rotation on a wind shear kind of day. Fascinating stuff for any pilot.

DP Davies book is fine too.

I know one TWA pilot who was called into the office for landing a 767 at KPHL with vref plus 25...he was just told to divert next time.


so its up to you...if you have to add more than 20 knots, divert or be a test pilot

[safetypee]

BOAC, JT, et al. Old, but not a really an old dinosaur …?
Certified landing performance does not include any margin.
Operational landing performance does have a margin, but its ‘use’ unfortunately has many varied views.
When and where the margin came from is far from clear. The matter was researched recently by the ICAO working group on runway friction without a definitive conclusion; strong contenders are RAE runway surface research (1960s -), wet runway braking and early FAA tests.
There is an interesting view associated with proportionality from innate human judgement; i.e. a one-to-one fight with an adversary should be not considered, whereas two-to-one looks much better. As this sequence is developed, the 1.67 margin looks about right!

However I digress; according to the UK CAA (defunct AIC ?), factored performance provides a safety margin for the normal variations in daily operations (normal not defined), but not predetermined excesses. Furthermore, while the margin might accommodate a speed up to Vref+15, it will not compensate for high speed / TCH or long touchdown simultaneously (this can be evaluated with modern rules of thumb, e.g. AC 91-79).

Overall the message is Vref+5 at TCH, on speed, and good TD position.
The margin is for human variability, not pre-planned deviation. We should not kid ourselves that we can control our variability.

Info: http://www.nlr-atsi.nl/downloads/analysis-of-existing-practices-and-issues-rega.pdf

http://www.nlr-atsi.nl/downloads/safety-aspects-of-aircraft-operations-in-cross.pdf

http://www.nlr-atsi.nl/downloads/crosswind-certification-how-does-it-affect-you.pdf

[safetypee]

rigpiggy, respecting your right to post previous experiences, it might be of greater value for a mixed experience audience to reflect on what might have been considered rather than relating an event which perhaps should not be remembered for its outcome.
What speed bleed-off should be planned, and when/where/how is this to be achieved?
How is landing distance calculated for such excess?
Would the nose wheel touchdown first; would the main wheels / squat switches remain in-the-air?
Do any retarding devices depend on the squat switches?
What if the plan did not proceed as expected; the wind changed; head/crosswind?
What if?

[BOAC]

Fellows - I was not advocating flying at Vref+20 per se, and as always, headwind component is ideally bled off approaching touchdown and gust factor maintained (according to 'judgement', of course). I feel sure that the 'limit' of 20kts that Boeing certainly publish is based partly on the 'historic' BCAR figure and also on the practicality of eliminating excessive speed in the last stages. Given that touchdown is ideally conducted at whatever 'Vref' is chosen (-5), as modified by 'factors' and that 100% of the headwind (ie twice your chosen additive) is reducing your LDR , I am pretty sure the system is fine and workable. Should you happen to have added 40kts for a 40kt gust factor and it is 'not there', it may not be your day, but isn't that life? Surely better that than to drop heavily out of the sky with a 40kt speed loss in the flare?

[safetypee]

BOAC, your ‘surety’ assumption about Boeing’s addition might be the weakness of your argument. However, I would accept that whatever a manufacturer publishes is normally based on sound reasoning.
An exception to the reasoned argument might be found in the FSF stabilised approach criteria (< Vref + 20). Most manufacturers preferred Vref+15, but Boeing’s argument that Vref+20 was already in their books won the day (he who pays the piper).

You could argue the probability of having a heavy landing vs a rough overrun; but my earlier question was if the industry has any recent atmospheric data to help validate procedures and additions based on the likely wind change (what is the extent of the threat), or if modern aircraft might be more resilient to wind shears and gusts during landing.

[Zoom-56]

Just to throw another bit of confusion into the mix, as a retired military test pilot and current glider flight instructor, I can confirm something that you will all appreciate: that a headwind reduces your geographic glide ratio. To compensate for this effect when on a glider cross-country, the rule of thumb is that for "most gliders" adding about half the steady-state wind to the no-wind speed for best aerodynamic glide ratio gives close to the best geographic glide ratio when fighting a headwind. Who cares about this when you have engines to get you there? No one really, but when the dust settles, having the extra knots of half the headwind may actually give you the lowest fuel burn for the full final approach (power setting is higher, but you spend less time driving). Of course, this is just conjecture on my part.