Speed reduction from Vref+20 to Vref+15 in Boeing

[agg_karan]

Dear All,
This is regarding the new speed reduction on (VApp)
On b777 max Vapp is Vref+15 (wind correction) now instead of Vref+20 which was there for several years.

I have been told it’s across b777/787/737 as well (not sure about the 747)

What is the reason behind this can anyone clarify ?
Thankyou & regards

[PEI_3721]

ak, I am unable to answer the question directly, but a good starting point is to ask why was Boeing’s previous value set at Vref +20, particularly when compared with many manufacturers recommendations of Vref +15

[ImbracableCrunk]

B737 has been changed to Vref+15 max.

[eckhard]

I seem to recall that Required Landing Distance calculations assume a max speed of Vref+15 at the threshold?
Maybe it was in BCARs?

[john_tullamarine]

Maybe it was in BCARs?

I'm really scratching the memory cells now .. but I vaguely can recall a requirement for a 20kt additive to be accounted in the landing distance pad under the BCARs. Would need to do some hunting around to find something to hang my hat on, though.

[Goldenrivett]

It's unlikely be a landing distance limit - why have a maximum Vref increment on a very long runway with a strong headwind?
It's more likely to be a body attitude limit at touch down which has an unacceptable risk of a nose wheel impact before main body gear.

[PEI_3721]

eckhard, JT, et al.
The U.K. CAA issued an AIC on landing performance, relating to Vref +15, this was published at the advent of JAR, after BCAR, thus it may have origins there
‘The Landing Performance of Large Transport Airplanes’
https://www.theairlinepilots.com/forumarchive/pilotslounge/landingperformanceoflargetransportairplanes.pdf

Although this document should now be superseded, with what and where remains unknown (CS-25 part 2 ?), yet the advice and background in the AIC may still be very relevant.

This view of max approach speed, Vref +15, was discussed during the formulation of the FSFs ‘Stabilised Approach’ criteria, but Boeing’s view, Vref +20 prevailed, apparently a commercial issue. The finally agreed position was that the conditions for a Stabilised Approach were only recommended (should vs shall), so other manufacturers and operators could use Vref +15; interestingly not a universal understanding - perhaps another economic position (not having to go-around so often).
Boeing’s position left them to decide when and how to reduce speed to the lower value required at the threshold; but that would be by destabilising due to the late deceleration.

ak, IC #3, is there any evidence that Boeing’s latest change is associated with any additional low speed alerting as suggested during discussion / investigations into the 777 Asiana and 737 THY landing accidents?
Do any Boeing aircraft have low speed audio alerting, other than as required for the approach to stall and stall warnings, and EFIS display?

[B737900er]

My current operator told us that Boeing recently reviewed the VREF+20 procedure and found that it gave NO additional manoeuvre margin than +15 did. So they changed it for commonality.

[eckhard]

Thanks PEI_3721,

The AIC to which you provided a link states that Vref +15 knots was the maximum assumed threshold speed used in the “reference method”. The pilot would be assumed to discontinue the landing if the speed was higher. Good advice, even today!

[john_tullamarine]

A small ‘hat-hook’

The more recent stuff is noted. However, going back, perhaps 40-odd years, I am quite certain that I read a requirement for a 20kt pad to be accounted. I have some rather old UK AFMs tucked away and I suspect there will be something to guide me in the AFM wordiology. When I can locate the two manuals I have in mind I will have a dabble and see what might come to light. It remains that the OEM concerned may have adopted the figure unilaterally but, in the absence of a certification imperative, why would an OEM do so ?

[ImbracableCrunk]

My WAG is that if you search AvHerald for overrun vs stall, you'll find where the threat lies. Maybe 5kts is an attempt at mitigation.

[LeadSled]

Folks,
Going way way back, the max Vref+20 limit was set on the B707, faster than this and it would be the nosewheel that touched down first.

Otherwise nothing to do with landing field lengths, and in SFAR 422b/FAR 25 not accounted for in landing field length, the logic being that the ground speed from having to deal with such winds meant that the distance was no more than the same weight at zero wind and Vref would produce.

The BCARs of the day had a quite different treatment, but the results were pretty much the same.
Tootle pip!!

[john_tullamarine]

The BCARs of the day had a quite different treatment, but the results were pretty much the same.

My imprecise recollection was that the BCARs had a requirement for 20kt accountability, although that may have been seen in the end AFM rather than the Design Standard specifically - until such time as I can dig through the archives, I am not able to resolve the problem.

Might you be able to cite those aspects of the BCAR which represented a "quite different treatment" to help reconcile the ageing memory ?

[Goldenrivett]

Hi LeadSled,
I agree with your conclusion.

This Boeing patent https://patents.google.com/patent/US5823479A/en says
"In addition to a maximum value, during landing, it is also desirable that the pitch attitude not exceed a minimum value. During approach, the main landing gear and nose gear of an airplane are lowered. Because the nose gear is not designed to withstand the impact of airplane touchdown, it is crucial to land the airplane so that the main landing gear makes contact with the ground first. The separation between the nose gear and the ground when the main landing gear contacts the ground is known as the nose gear contact margin. In order to avoid nose gear first landings, a minimum pitch attitude, commonly referred to as a most nose gear critical flight condition, is defined. Landing at pitch attitudes equal to or greater than the minimum value for the most nose gear critical flight condition ensures that the main landing gear touchdown precedes the nose gear touchdown by a predetermined value. The predetermined value is known as the nose gear contact margin requirement. This requirement is also in degrees and is defined by the angle between the runway and a line extending between the main landing gear and the nose gear on the airplane."

[LeadSled]

The BCARs of the day had a quite different treatment, but the results were pretty much the same.

My imprecise recollection was that the BCARs had a requirement for 20kt accountability, although that may have been seen in the end AFM rather than the Design Standard specifically - until such time as I can dig through the archives, I am not able to resolve the problem.

Might you be able to cite those aspects of the BCAR which represented a "quite different treatment" to help reconcile the ageing memory ?

JT,
Without digging into very old boxes (even if I could find them), it goes something like this:
The FAA landing field length was the demonstrated length with Vref at 50', and that became 60% of the published landing field length. ie: Actual factored by 1.67.
The BCAR approach was to "account" for "pilot technique" by postulating a dispersion of threshold speed speed, up to +20, (and, if I recall correctly, height) but then only have a much smaller factor, so that the resulting published landing field length was very similar to the FAR 25 result. Certainly "close enough for Government work".
Don't quote me, but as I recall, the BCAR factor was something like 15%, all engines in the normal landing configuration.
Tootle pip!!

[john_tullamarine]

I think I would have to opt for a contrary tale to that, good sir.

No problem with the FAA approach, but that wasn't in contention anyway.

The approach suggested in your post for the UK's BCARs would result in a somewhat lesser landing distance margin to a first approximation, especially for the higher jet speeds where the margin would decrease with increasing Vref values. More to the point (and I would, likewise, have to excavate the archives to dig out the documents to be 100% sure) I have done quite a bit of performance work (in the distant past) on a variety of BCAR AFMs and, as I recall, they all had a 1.67 factor for normal scheduled landing and 1.43 for alternates ? 1.15, more likely, is to be seen for small GA aircraft ?

[LeadSled]

I think I would have to opt for a contrary tale to that, good sir.
No problem with the FAA approach, but that wasn't in contention anyway.
?
JT,
It is a long time ago now, but probably the only reason I mention it, is that it was a prominent part trans-Atlantic certification wars of the day, still not settled to this day. The long running US-EASA "Harmonization" program is and has always been beset with national rivalries, not unknown here.
One of the major objections was the repeatability of the UK test regime, versus FAA, and what was the point, if the answer was the same, more or less.
The answer probably lies in what EASA these days call TGLs.
Tootle pip!!

[john_tullamarine]

I think we shall have to agree to disagree on the odd point.

[PEI_3721]

Having experienced interesting and wide ranging views on Vref + 20, perhaps the original issue of Boeing’s change to Vref + 15 might be clarified.

So far there are suggestions of ‘no benefit’ from a previous value, #8, which cynically could be the ‘lawyers’ reduction of exposure to overrun accidents, as #11. Alternative reasons range from modifications, to having new data.

Can anyone comment if the latest aircraft variants are more ‘slippery’ on the approach - take longer to slow down. How long (time distance height) would it take to reduce speed from +20 to +15, (or nearer Vref) to establish a stabilised approach before 500ft - using a realistic procedure?

Are there any modifications associated with the recommended new max approach speed?
Is the change a crew/training manual amendment or a special notification; was there any accompanying explanation?
Are these changes linked to the use of FOLD for landing data - any changes to tables of ‘actual’ landing distances, thus coming in line with FAA (world) recommendations?

It’s not only interesting to know the reason behind the change, but also to be aware that with change there is opportunity for new risks to emerge from the change itself; someone somewhere might get ‘it’ wrong. Wrong version, wrong time place etc; knowing why might just help avoid those situations.