PDA

View Full Version : Setting Vref Approach speed 737


David1991
21st Jun 2019, 11:40
Would like to know how you handle setting the Vref Speed for the Approach. Are you Talking the current gross weight or overwritting it with the estimated? What are the advantages/disadvantages? Yesterday we had the Issue with the PFD showing a different Vref speed compared to the one in FMCs Approach page. How can this Happen?

thanks!

172_driver
21st Jun 2019, 11:52
The Vref may change a knot or so between the brief and final approach. If you're unhappy with one knot difference then just update the PFD speed. Takes 2 button pushes. My opinion.

Centaurus
21st Jun 2019, 12:46
See the FCTM. If manual throttle, don't forget to bleed off the half the steady headwind component approaching touchdown that was added to VREF as well as the 10 knots added to VREF if ice additive. The latter 10 knots is reduced once below 200 feet.

Themax23
22nd Jun 2019, 10:10
I leave the current gross weight. Will only change 1 or 2 kts from TOD to landing. Much safer to fly 1kt faster than to risk entering a wrong weight.

Eric Janson
22nd Jun 2019, 12:05
The indications on the Speed Tape are for the aircraft weight at that moment.

FMC calculations are based on aircraft weight at the end of the inserted route.

In the event of an immediate return the route has to be cleared before looking at Vref otherwise it will be incorrect.

Themax23
22nd Jun 2019, 14:37
The indications on the Speed Tape are for the aircraft weight at that moment.

FMC calculations are based on aircraft weight at the end of the inserted route.

In the event of an immediate return the route has to be cleared before looking at Vref otherwise it will be incorrect.

That's not true. Where did you get that info from ?

Banana Joe
22nd Jun 2019, 21:33
The FMC calculations are based on actual GW. The GW can be overridden with the estimated GW at landing to get Vref speeds to be selected for the estimated GW.

wiggy
22nd Jun 2019, 22:56
The Vref may change a knot or so between the brief and final approach. If you're unhappy with one knot difference then just update the PFD speed. Takes 2 button pushes. My opinion.

True...FWIW a slightly different Boeing in question but FWIW our training department/management has just decreed, in the interests of minimising FMC punching after TOD, that one should select Vref at the briefing stage and then barring any major events such as a go-around leave it alone...

galdian
23rd Jun 2019, 06:48
FWIW where I am at present the F/O's are taught they MUST override the expected landing weight and set that Vref speed.
On occasion I point out to them, on final approach, that the FMC approach speed is actually 1 kt FASTER than the Vref they set due track shortening, damned fine efficient descent profile....whatever.

No hassles, just reselect the correct approach speed.

After landing I just discuss what would happen IF an accident occurred and in evidence it was found the crew were KNOWINGLY using an incorrect Vref setting for the actual landing weight??

Suppose whether the argument from the prosecution or defence in the courtroom!

Cheers.

safetypee
23rd Jun 2019, 08:47
Requiring the speed to be checked and reset should trigger the reassessment of the speed required according to the landing conditions - operational requirement EASA, recommendation FAA.
FMC might have the forecast wind, but not actual or gusts / turbulence, which could require a higher speed.
The FMC would not have runway condition or braking action (unless entered), which should also influence speed selection or at least speed or configuration management. Which in turn should trigger the need to adjust landing performance / auto brake selection. Also act as a cross check of landing weight against the expected or computed value.
A good triggering procedure should force engagement of thought, opposed to blind following of some computed value.
So not having a display of computed value might be better.
Just because something can be done is not always a good idea, or because it reduces workload; we often fail to consider both technology accuracy - reliability, and human performance in our choice of action

David1991
23rd Jun 2019, 12:07
So in Order to set the Target Speed.. which Vref Shall i Look to.. the one on the pfd or the one on the Fmc... theY differed Recently... that s why i am asking

172_driver
23rd Jun 2019, 13:06
So in Order to set the Target Speed.. which Vref Shall i Look to.. the one on the pfd or the one on the Fmc... theY differed Recently... that s why i am asking

The one displayed in the FMC is the correct one, to the best of your knowledge.

172_driver
23rd Jun 2019, 13:08
FMC calculations are based on aircraft weight at the end of the inserted route.

In the event of an immediate return the route has to be cleared before looking at Vref otherwise it will be incorrect.

​​​​​​​That is not correct, is it?

safetypee
23rd Jun 2019, 13:22
So in Order to set the Target Speed.. which Vref Shall i Look to.. the one on the pfd or the one on the Fmc... theY differed Recently... that s why i am asking

The glib answer is to use the ‘book’ value, the AFM is the basis of certification. Thus what is the basis of the FMS speed, computation, integrity level; or the basis of EFIS; where do the values come from.
Without knowledge of your aircraft, if the EFIS Vref display is automatic and associated with low speed awareness and stall (amber / red bands), normally based on AoA (certification basis), then this should be the more accurate (less susceptible to error) because AoA does not depend on wt. Alternatively is this the manual set speed via the FGC.
The FMS could use AoA, but from the discussion so far appears not, because the interface with wt.

yoko1
23rd Jun 2019, 16:09
A whole lot of hand wringing going on for what may be a knot or two. Do you "really" know what your aircraft weighs at any particular moment? Can you honestly hold your approach speed to that precision? Do you know exactly what the wind will be in the flare?

The FMC also produces a stab trim calculation to the hundredth digit. Can you even set it? Did you know that this setting is almost always wrong?

We used to have a saying at a previous employer about their approach to certain tasks: "Measure it with a micrometer, mark it with a grease pencil, cut it with a blow torch." This discussion kind of reminds me of that.

Yes, be close to approach speed when you start your flare. However, just as important: land in the touchdown zone, on centerline, no sideways drift, and (very, very important), throttles closed. Having your power up chasing a speed when you touchdown can cause you a world of hurt.

Bonus question: On the 737, would you rather have a crosswind from the left or the right? Why? How can you use that knowledge to your advantage with the wind straight down the runway?

safetypee
23rd Jun 2019, 16:51
yoko1, similarly, how would you know if there was a gross error in one or the other value. Thus the need to know how each are calculated and which should have the greater integrity.
As with any 2 value, dual system, if there is an error between them, then the correct value cannot be found without a third source for comparison or a very good understanding of the origin of each value. e.g. obvious error in FMS wt.

yoko1
23rd Jun 2019, 18:03
yoko1, similarly, how would you know if there was a gross error in one or the other value. Thus the need to know how each are calculated and which should have the greater integrity.


My point is that each of these sources are calculated values and can only be as precise as the inputs into those calculations. I've had my share of misloaded aircraft over the years, so it is advisable to apply an appropriate amount of vigilance to the FMC value. With enough time in the aircraft, a pilot should eventually get a sense whether the FMC calculated value that they are attempting to fly is appropriate for the conditions. Fortunately, the 737's I operate have a direct reading of the AOA vanes displayed on the PFD which provides a good sanity check for one's approach speed. That being said, this aircraft is far more tolerant of being a few knots fast than a few knots slow.

None
23rd Jun 2019, 20:27
Fortunately, the 737's I operate have a direct reading of the AOA vanes displayed on the PFD which provides a good sanity check for one's approach speed. That being said, this aircraft is far more tolerant of being a few knots fast than a few knots slow.

I agree with yoko1. And here's an example of the AOA indicating to me that the final approach speed is too slow for my comfort level. How would I know without cross-checking the AOA?


https://cimg3.ibsrv.net/gimg/pprune.org-vbulletin/480x640/aoa_flaps_30_at_flaps_40_speeds_2_aeae7e803812cfbd744b0ec40e b36f8c4fae4aaa.jpg

RVF750
24th Jun 2019, 13:19
My favourite speed for some reason....

Answer to Yoko is always land on the left wheel if you want a smooth touchdown. In our company we are required to brief before TOD and complete the descent checklist. The Assumed GW at touchdown is taken from the FMC progress page where you look at the difference between current and predicted. It's usually about a tonne. Then you go to the Init Ref and deduct this difference from current GW and double tap the speeds for the flap setting. You can pop in plus 5 on the next line or more if you wish, though it's just as easy to manually set that on getting the touchdown wind from ATC.

This flying game is not rocket science, yet youngsters are taught so much by rote now that they hang their hats on flying numbers at all times. It's a world of butt covering and legal minefields. and I'm counting the days till I can retire. I blame the culture on Boeing and Airbus lawyers and the dumbing down it entails. I've never flown a type where I was taught and expected to know so little about the airframe!

So yes, left side first. It needs a minute or two of reading the WOW sensor logic to understand why, and the number of blank and confused faces I've had alongside me who can't understand it after four hours of trying never ceases to amaze me. They're the future and single pilot aircraft will be designed for them to fly, not the likes of me. That really scares me!

misd-agin
24th Jun 2019, 14:15
I agree with yoko1. And here's an example of the AOA indicating to me that the final approach speed is too slow for my comfort level. How would I know without cross-checking the AOA?


https://cimg3.ibsrv.net/gimg/pprune.org-vbulletin/480x640/aoa_flaps_30_at_flaps_40_speeds_2_aeae7e803812cfbd744b0ec40e b36f8c4fae4aaa.jpg

The green band is ‘on speed’ (“Approach Reference Band”) for “appropriate range of approach AOA for a Vref + 5 approach.

It’s Vref +7 in your example even at the higher AOA limit of the ‘on speed’ range.

The green band band covers a significant speed range and isn’t as accurate as military AOA ‘on speed’ green donuts. Because of that range/inaccuracy (roughly 2.5 units) it’s not used as a primary performance instrument.

yoko1
24th Jun 2019, 21:19
This flying game is not rocket science, yet youngsters are taught so much by rote now that they hang their hats on flying numbers at all times. It's a world of butt covering and legal minefields. and I'm counting the days till I can retire. I blame the culture on Boeing and Airbus lawyers and the dumbing down it entails. I've never flown a type where I was taught and expected to know so little about the airframe!

So yes, left side first. It needs a minute or two of reading the WOW sensor logic to understand why, and the number of blank and confused faces I've had alongside me who can't understand it after four hours of trying never ceases to amaze me. They're the future and single pilot aircraft will be designed for them to fly, not the likes of me. That really scares me!

Pretty much my thoughts exactly.

Last point on the landing the 737, an issue to which I alluded earlier. You must, must, must have the throttles closed at touchdown! The spoiler deployment logic looks at throttle position, wheel spin-up, and weight-on-wheels. If you touchdown with throttles out of idle, you may get wheel spin but no spoiler deployment thus setting the aircraft up for a bounced/skip landing. As the aircraft rises, the tendency is to pull the throttles to idle. The wheels are still spinning, so now you get your spoilers! The aircraft sinks fast so the flying pilot may pull back on the yoke setting up a tail strike. BAM! Any significant bounce, add power smoothly and execute a go-around.