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Author Topic: Min maneuver speed change-over at 20000 ft  (Read 1984 times)

Hardy Heinlin

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Min maneuver speed change-over at 20000 ft
« on: Mon, 5 Oct 2020 17:33 »
Good evening,

this subject might be company specific.

As we know, above 20000 ft the green PFD flap speed bugs are removed. Also, above 20000 the flap-up amber band (1.3g/1.2g maneuver margin) refers to the low speed buffet, below 20000 to stick shaker speed.

PSX calculates the flap-up amber band based on a special graph from a certain document. On that graph there's no abrupt change at 20000 ft.

Has any 744 driver ever noticed a small jump of the amber band when passing 20000 ft, i.e. when changing the maneuver reference from low speed buffet to stick shaker speed?

And: Can anyone confirm the amber band may be higher than the flap UP bug when the aircraft is close to 20000 ft?

Thank you.


Regards,

|-|ardy

andmiz

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Re: Min maneuver speed change-over at 20000 ft
« Reply #1 on: Tue, 6 Oct 2020 00:58 »
Hi Hardy.

I haven’t seen it jump, but will look a bit more closely next time.  A few more things; our manuals say “approximately 20,000ft”, so it may not be a hard and fast threshold.

We also do not get a flap UP bug until we are below 20,000ft.  Operationally, this means that when accepting a speed reduction above 20,000ft, we need to be conservative on what speed we will accept to ensure that it will be above the UP bug when we pass below 20,000ft.

Hardy Heinlin

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Re: Min maneuver speed change-over at 20000 ft
« Reply #2 on: Sun, 17 Jan 2021 22:58 »
Refreshing this discussion ...

For reference, the 744 FCTM says:

Quote
Below approximately 20,000 feet, the flaps-up amber band will not exceed the “UP” speed bug at VREF30 + 80 knots. [...] Above approximately 20,000 feet, the top of the amber band shows the speed that provides 1.3g maneuver capability to low speed buffet [...]

I just learned how the 744 sims at Lufthansa behave: In climb, at 20000, the top of the amber band "jumps up" to the 1.3g reference.

My interpretation: In climb, from the ground up to 20000, the top of the amber band rises more and more (as we know), but not above the FLAPS UP marker. At 20000, the flap speed markers disappear (as we know), and the top of the amber band jumps up (no gradual motion).

Has anyone noticed such a jump on the real aircraft? (744, 777 ...)


|-|ardy

simonijs

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Re: Min maneuver speed change-over at 20000 ft
« Reply #3 on: Mon, 18 Jan 2021 13:24 »
I can only think of two reasons for that jump:

1 - The following used to be the text in KLM's AOM (Volume II - with page date 1 JAN 1998): "The top of the amber band along the speed scale on the PFD is the minimum manoeuvring speed. Above 20.200 ft it is the speed at which the airplane has a 0.3 g manoeuvre margin to initial buffet; Below 20.200 ft, it is the speed at which the airplane has a 0.14g manoeuvre margin to the 1 g stick shaker speed." Obviously, that would have produced a jump. However, in the current FCOM that last part has disappeared. The FCOM wording now is: "Top of the amber bar (...) provides: 1,3g maneuver capability to stick shaker or VREF+80, whichever is less, with flaps up at or below approximately 20,000 FT; 1,3g maneuver capability to low speed buffet with flaps up above 20,000 FT." No reference to that 0.14g manoeuvre margin anymore. And I guess Boeing has removed that distinction in an update of the amber band calculations after 1998.

2 - Looking at KLM's Buffet Onset Boundary Chart [BOB] for an aircraft at 390 metric tons, Low Speed Buffet onset occurs at ± M0,505/± 245 KIAS at 17000 ft. At 20.000 ft: buffet onset occurs at ± M0,533/± 247 kts. From the Stall Speed charts*, a Pax/Combi aircraft at 390 MT will stall at 213 KIAS. With the Red Band set to the stick shaker activating speed (which is at least 5 % prior to reaching the actual 213 KIAS 1G stall speed), the stick shaker would likely activate at 224 KIAS, so the Red Band would sit at ± 224 kts. Above 20.200 ft the red band is now indicating with reference to Low Speed Buffet Onset, which at 17.000 ft is determined to be at 245 kts. That is an approximate 20 kts jump of the Red band upon passing 20.200 ft; and since the amber band is calculated as a margin on top of the Red Band, that one will jump as well.

I am now voting for this second option. The Red Band is leading, within KLM's fleet the amber band is calculated as a 1,3G margin on top of that (using TAS values) whether it is calculated from the stick shaker of from the low speed buffet reference.

Regards,
Simon

* There is a note on the Stall Speed Chart: "Based on most forward CG, and applicable to Take off and Landing altitudes only." The lowest readable values for Mach-number and KIAS on the Buffet Onset Boundary chart are for an aircraft at 400 MT and 16.000 ft/390 MT and 17.000 ft. KLM/Martinair Cargo 747s depart from Quito (7910 ft) and Bogotá (8358 ft) on a regular basis albeit not with these high weights...

Hardy Heinlin

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Re: Min maneuver speed change-over at 20000 ft
« Reply #4 on: Mon, 18 Jan 2021 15:30 »
That would mean that the red band jumps also.

I've learned that there are training exercises in the sims that demonstrate the "amber band jump". I haven't heard of a "red band jump" yet. That needs to be confirmed ...


Regards,

|-|ardy


By the way, re 20000/20200 ... I guess this 200 feet gap in the text is a hysterisis to avoid flicker when flying level at 20000. So in climb: 20200. In descent: 20000.
« Last edit: Mon, 18 Jan 2021 17:30 by Hardy Heinlin »

simonijs

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Re: Min maneuver speed change-over at 20000 ft
« Reply #5 on: Mon, 18 Jan 2021 18:57 »
Normally you won't see the amber band while climbing above 10.000 ft and accelerating to ±320 kts, so you won't notice such jumps. There is another (special) document for KLM's flight crew that deals with high elevation airports. For instance Quito: it is common practice to set 250/18000 as a SPD RESTR on the VNAV CLB page to avoid acceleration above 10.000 ft while still in between mountains, at least for PAX flights with a 777. That 777-FCOM, however, has the same wording with respect to top of the amber band above/below 20.000 ft as can be found in the 747 FCOM.

A friend of mine was going to fly to Quito today on a 777, and I have asked him to change that SPD/RESTR to 250/21000 ft on the outbound flight to Guayaguil - if that would be safe to do - and check for any visible jumps. Well, he was taken off that flight and will do another destination tomorrow. But he will get there some day and then it also depends on his captain, willing to cooperate...

Regards,
S.




simonijs

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Re: Min maneuver speed change-over at 20000 ft
« Reply #6 on: Wed, 20 Jan 2021 10:27 »
Hi Hardy,

One more thought: yesterday I found some dust-covered lecture notes provided by a former KLM MD-11 Flight Performance Engineer. He states that the stall speed in KIAS - for large transport category aircraft - remains constant with altitude up to 12.000 ft. Above 12.000 ft, and due to the effects of compressibility, the stall speed will increase by approximately 2 kts per 1000 ft.
Then a 390 ton aircraft, with a stall speed of 213 kts (below 12.000 ft) will stall at approximately 229 kts at 20.000 ft. With the stick shaker set at least 5 % above that speed, the red band would sit at ± 240 kts. If the red band then indicates with reference to Low Speed Buffet above 20.200 ft, a jump (if at all...) would only occur over ± 5 kts. And maybe, Boeing has set it to 6 or 7 %, avoiding jumps.

A jump should then also be visible descending through 20.200 ft, although weights of course are much less and the red band - most likely - won't be visible anymore. But the top of amber will still show, I guess. Perhaps somebody is willing to look for such a jump of the amber band on the real deck.

There is more reading to be done on this subject...: 12.000 ft in these lecture notes, 15.000 ft in formulas that I use or 20.200 ft as indicated by Boeing. And I need to clean the house!

Regards,
Simon


Hardy Heinlin

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Re: Min maneuver speed change-over at 20000 ft
« Reply #7 on: Wed, 20 Jan 2021 10:41 »
Hi Simon.

With the stick shaker set at least 5 % above that speed, the red band would sit at ± 240 kts. If the red band then indicates with reference to Low Speed Buffet above 20.200 ft, a jump (if at all...) would only occur over ± 5 kts. And maybe, Boeing has set it to 6 or 7 %, avoiding jumps.

I don't have my Boeing sources at hand right now, but some years ago I modified the physical stall speed and the red tape in PSX according to some Boeing graphs which revealed that the gap between the red band and the actual stall increases very much with altitude. I think the gap extends to over 20 kt at high altitudes (I don't have the values in my head). I'd like to keep that red stuff as is, i.e. as documented (no theories required). I would just like to get a confirmation about that amber band jump. Maybe in two weeks ...


Regards,

|-|ardy

74pilot

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Re: Min maneuver speed change-over at 20000 ft
« Reply #8 on: Wed, 20 Jan 2021 14:56 »
It definitely "jumps". It's mainly noticeable when descending, where ATC gives a speed reduction. Of particular note when we fly into MEX, where the elevation is around 7600' AMSL. We can be quite heavy at times, carrying extra fuel, and often, they want us to reduce speed early. 200 KIAS at FL200 can be an issue, until the speed jumps. But also as we can't take flaps above FL200.

I am not sure but have always thought it had to do with compressibility. B747 uses KIAS while I believe the 777 (maybe only some of them) uses KEAS. This is why the stall speed is higher at altitude. If it wasn't for compressibility, the indicated stall speed should be the same for all levels (same weight and other factors).

Compare with the stall speed table for a 777.

If I remember, I shall make a video of the speed tape, next time I descend below FL200. It should be similar on the -8 and the -400.

There is another interesting thing to look at, the minimum holding speed on the holding page.

Also, the ref speed changes when you select F25 of F30 on the CDU. It's weird, but some times, if a certain flap speed is just a few knots above what ATC requires, changing from F25 to F30 on the APPROACH CDU can fix it. Yes, it's only theoretical, because how should the final flap setting for landing influence the minimum speed while you are only at, say, flap 1?

Example: Minimum flap speed at a given weight and flap setting is 223. ATS wants 220 knots, but you want to delay further flap a bit. PM selects F30 on the CDU and minimum flap speed changes to 217 ...

simonijs

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Re: Min maneuver speed change-over at 20000 ft
« Reply #9 on: Wed, 20 Jan 2021 18:06 »
Hi,

Well, if you ever get there again and you were to depart on a SID towards the southwest...: at some point it says 250/18000A. If that 250 kts could be maintained until above 20.200 ft and perhaps with the red band still visible: will you see that red band jump as well?

For me, part of the fun is the theoretical part of aircraft performance; calculation of minimum speeds, experimenting to find required coefficients etc. happened to be what I was doing last month.  :)

Regards,
Simon

Hardy Heinlin

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Re: Min maneuver speed change-over at 20000 ft
« Reply #10 on: Wed, 20 Jan 2021 21:10 »
Example: Minimum flap speed at a given weight and flap setting is 223. ATS wants 220 knots, but you want to delay further flap a bit. PM selects F30 on the CDU and minimum flap speed changes to 217 ...

This effect refers to an airline option (also available in PSX). I've learned that British Airways used that option. Or maybe vice versa; all other airlines used it. I need to refresh my memory.

Anyway ... the one option is this:
• Flap speeds always refer to VREF30, even when a flap 25 REF is entered (25/...)

The other option is:
• Flap speeds refer to VREF25 when a flap 25 REF is entered (25/...)

In PSX you can select this on Instructor > Model > FMS with the checkbox "Flaps speeds refer to VREF30".


|-|ardy
« Last edit: Wed, 20 Jan 2021 21:34 by Hardy Heinlin »

74pilot

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Re: Min maneuver speed change-over at 20000 ft
« Reply #11 on: Thu, 21 Jan 2021 00:25 »
Hi,

Well, if you ever get there again and you were to depart on a SID towards the southwest...: at some point it says 250/18000A. If that 250 kts could be maintained until above 20.200 ft and perhaps with the red band still visible: will you see that red band jump as well?

For me, part of the fun is the theoretical part of aircraft performance; calculation of minimum speeds, experimenting to find required coefficients etc. happened to be what I was doing last month.  :)

Regards,
Simon

We don't need an ATC speed restriction for that. Often there is a requirement to make a certain level by a certain waypoint, typically when an airport is close to a different FIR or ATC area. We often have to reduce the speed from the calculated optimum, to get a higher climb angle. At high take-off weights on the 747-8F, optimum climb speed can be 360 knots, and often the speed is manually changed on the VNAV CLB page to give more buffer towards overspeeding. But at lower weights, optimum climb speeds is much lower.

I will try to remember to have a look at both. At the moment, we are usually quite heavy on most flights. Shorter sectors with less fuel obviously means lower climb speeds, so I will have a look on one of those, as we pass FL200.

Hardy Heinlin

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Re: Min maneuver speed change-over at 20000 ft
« Reply #12 on: Sat, 23 Jan 2021 01:21 »
OK, I just learned from a real flight: A 28 knot jump of the amber band. The red band doesn't jump.


|-|ardy

simonijs

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Re: Min maneuver speed change-over at 20000 ft
« Reply #13 on: Sat, 23 Jan 2021 11:51 »
Interesting ! Was that at MTOW?

If only the Amber Band jumps and the Red Band doesn't, then that might indicate it is still calculating with 1.14G up to ± 20.000 ft and with 1.3G above that altitude... All in all, 28 kts is a lot (my formulas only show ± 21 kts at 400.000 kgs and 20.000 ft, when I change 1.14G into 1.3G).

Regards,
S.

Hardy Heinlin

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Re: Min maneuver speed change-over at 20000 ft
« Reply #14 on: Sat, 23 Jan 2021 12:17 »
396 tons.

In climb, it changes at 20200 ft. I guess, in descend it changes at 20000 ft.


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dhob

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Re: Min maneuver speed change-over at 20000 ft
« Reply #15 on: Mon, 25 Jan 2021 08:31 »
Modern Boeing airplanes are stall tested over a range of altitudes, and published for these altitudes. General stall speed increases approximately 2-3 knots per 10000 feet.
The amber band reflects 1.3 to Vss (stick shaker) below 20000 feet. (FAA allows operators either 1.2 or 1.3g, our FMC is set to 1.3g). It is not allowed to be less than 1.2g.
Airplanes certified after 1985 use Vs1g as stall speed, or the speed at which the airplane can no longer maintain 1g level flight.
Stall at current speed (accelerated stall) is (Vs)^2=n(Vs1g)^2.
This can be used as margin to Vss as well, (Vs)^2=n(Vss)^2.
Or Vs=sqrt(n) (Vss)
Since n, or load factor for the amber band is 1.3g, then Vs=sqrt(1.3)Vss, or Vs=1.14Vss.
Therefore, in 1g level flight, the speed of top of the amber band is 1.14 times Vss.
Also, Vss=1.05(Vs1g) and VREF = 1.23Vs1g.
From this all the speeds can be derived. At max landing weight, VREF is 156 knots.
156/1.23= Vs1g = 127.
Vss = 1.05Vs1g = 133.
Top of amber band in 1g flight, Vs=1.14Vss = 1.14(133) = 152.

simonijs

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Re: Min maneuver speed change-over at 20000 ft
« Reply #16 on: Mon, 25 Jan 2021 12:35 »
Hi dhob,

I agree with your way of calculating the 1,3G maneuver margin from the stall or stick shaker speed. But two notes, if I may...:

- All stall speeds are calculated based on True Airspeed and have to be converted into Indicated Airspeed; values for TAS and IAS may be the same in a sea level environment, but will be quite different at higher elevation airports or at FL350. The 1,14 ratio between these speeds will change (slightly) as well.
- your example applies to landing configurations and is based on VREF 30 (VREF equals V2). Not a configuration that you would likely see when passing 20.000 ft. So it doesn't answer the question why a jump of the amber band would occur around this altitude while in a clean configuration. If the amber band jumps while - apparently - the red band remains where it is, the calculation of the top of the amber band must change at this jump.

Boeing makes a distinction at some 20.000 ft: reference to Stick Shaker below 20.000 ft, reference to Low Speed Buffet Onset above 20.000 ft. Then a jump must have to do with this changing reference.

And this may be a really stupid question...: does the stick shaker activate above 20.000 ft, approaching the red band? I am quite sure it does, forcing the pilot to reduce the angle of attack. But then why make this distinction? I can only think of a difference between stick shaker speed (based on stall speed) and stick shaker speed (based on low speed buffet onset). But if they are different, then why doesn't that red band jump as well...? 

Regards,
Puzzled Simon


dhob

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Re: Min maneuver speed change-over at 20000 ft
« Reply #17 on: Wed, 27 Jan 2021 08:59 »
The Stall Warning Management Computer (SWMC) calculates the stick shaker speed. The FMC calculates the initial buffet onset speed, Vbflo. When the flaps are up, Vmin (top of the red band) is the higher of Vss or Vbflo.
Above 20000 feet, Vbflo can exceed Vss. especially at heavier weights. Below 20000, Vbflo is scaled toward zero, as such Vmin is equal to Vss.
The minimum maneuver speed is defined in the FMC as an envelope of 1.3g margin to initial buffet, both low speed and high speed. However, much like Vmin described above, below 20000 feet, 0.3g to initial buffet is always more limiting than 0.3g to Vss. Since the buffet limit is washed out at a constant rate below 20000, the 0.3g to initial buffet margin is washed out with it.
When flaps are not up, the minimum maneuver speed is always 1.3g to Vss.
When flaps are up, below 20000 feet, the minimum maneuver speed is 1.3g to Vss, or VREF30+80 knots, whichever is less. This allows the airplane to remain in VNAV when slowing to flaps up speed.
Above 20000 feet, minimum maneuver speed is 1.3g to initial buffet. Note that even though the FAA allows less than 1.3 g margin, down to 1.2g, this only has effect above M.75 (high altitude). When below 20000, it's 1.3g to Vss regardless of what AMI value is set.
This concept is evinced clearly in the 0.3g margin to initial buffet diagram in some proprietary documents, and the margin to initial buffet does show a jump at 20,200 feet based on weight. At 600000 lbs. there is a very small jump. At 850000 lbs. the jump is fairly significant (30 knots approximately). Below 600K, no jump is evident.
Stall speed is not involved in this discussion. Vs1g is approximately 5 percent less than Vss, but that relationship doesn't apply to Vbflo.
These are the only factors that affect stall speed, or more precisely the maximum AOA at 1g: the center of gravity; the thrust; altitude; aeroelastic effects; rate of entry; and load factor.
Altitude does affect stall speed, but from sea level to 20000 feet the stall speed only increases approximately 4-6 knots.
The relationship described in a previous post of Vs=sqrt(n)(Vxx) holds for either 1.3g to Vss, or Vbflo, as such 1.14 times the Vmin speed is the minimum maneuver speed, or top of the yellow band.
It is my assumption the Boeing limitation to not extend flaps above 20,000 feet is directly related to this discussion in some fashion.

simonijs

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Re: Min maneuver speed change-over at 20000 ft
« Reply #18 on: Wed, 27 Jan 2021 17:23 »
Thank you for this detailed explanation.

At home I can look into Aircraft Operations Manuals [AOM], in use with KLM until ± 2006. In AOM Volume III, a Buffet Onset Boundary Chart [BOB] and two 1G stall speed charts (Pax/Combi and Freighter) can be found. BOB allows me to check Low Speed Buffet Onset values in terms of Mach-number and IAS: for highest weights from 15.000 ft and above; for weights < 340 metric tons [MT]: 20.000 ft and above. The lowest Mach number on BOB is 0,5 and the highest equals MMO.

In Excel, I created several spreadsheets that quickly calculate some desired value. It is then a matter of experimenting to find coefficients that make my Excel calculations match results, that are read off these AOM graphs; for all weights and altitudes. One of these sheets is calculating low speed buffet. The results, calculated by Excel, are within 1-2 knots (or less than 0,003M) of the values that are read off BOB. For all weights, at 15.000 ft or above.

Stall speed charts allow me to calculate the 1G coefficient of lift (CL), which turns out be ± 0,99-1,01 with Flaps UP. For this I am using TAS, that – once converted into IAS – match the numbers from the 1G stall charts. Pax/Combi numbers are slightly higher than those of the freighter. Adding 5 % to the speeds found, I can then compare numbers for VSS and VLBO.

VSS and VLBO differ quite a bit at 20.000 ft, VSS always being the lower speed. At 270.000 kg (600.000 lbs) I found a difference of 20 kts between the two. At 395.000 kg (870811 lbs) that difference is ± 24 kts. Even if the charted values were for MSL, and those 4-6 kts were added to account for the climb to 20.000 ft, the difference would still be significant. Slightly higher jumps are then calculated for the 1,3G margins (± 27 kts at 395 MT).
 
Another important difference is the value for CL. At VSS that value would have been ± 0,93 (395 MT). At VLBO, CL is ± 0,75 (still on the linear part of the lift curve): the aircraft isn’t stalled yet, it is just flying at a high Angle of Attack [AoA]. Because of this high AoA, air flowing over the top side of the wing is accelerated to the point where it locally reaches M=1, causing buffet (and perhaps trouble).
Is that the same buffet you were referring to in your last post: “Above 20000 feet, minimum maneuver speed is 1.3g to initial buffet”? Or were you referring to the buffet, that starts as the lift curve slope would begin to flatten out? All of my books address high speed buffet, none of them deals with low speed buffet. Just some internet sources (Skybrary, Flightliteracy…).

If VSS and VLBO indeed differ by so much around 20.000 ft, I would expect to see the red band jump as well. But apparently it doesn’t, as Hardy reported back. Still puzzled by that! Or still missing the point...

Regards,
Simon

Hardy Heinlin

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Re: Min maneuver speed change-over at 20000 ft
« Reply #19 on: Thu, 28 Jan 2021 01:06 »
... I would expect to see the red band jump as well. But apparently it doesn’t, as Hardy reported back. Still puzzled by that!

The EFIS symbol generator software may contain an interpolation function that transfers the red band slowly, smoothly from one mode to the other when the aircraft is in the upper sector above 20000.

Many features on the speed tape are smoothed out by the EFIS software. Even the grey speed tape itself has an artificial inertia (which is even relevant to V1 discussions); the lag is very small, though. It's just half a second or so. Anyway, smooth, gradual indications are not unusual.

When you're flying at very low airspeeds, I think such a jump of the amber band is acceptable, but a similar jump on the red band would be rather shocking, in my opinion. Imagine, the pilot has been in a gradual transition phase for some minutes, and suddenly the stick shaker starts, just because a software switch changed a design mode. There is no such step in the natural atmosphere. If the step is a matter of aircraft design rather than natural physics, the designers can decide themselves to make the transition slow or not. So why not make it slow? I would make it slow.

Here are some values from the afore-mentioned real-world example:

19500 - flapup:264, amber:264, red:253
20140 - flapup:264, amber:264, red:250
20220 - flapup:264, amber:292, red:250


Regards,

|-|ardy