737: Anti-ice -> Icing logic -> Minimum speed reversion on final

[STBYRUD]

Had my recurrent sim session recently, something strange happened during one of the approaches. Flap 30, pressurisation problem, so standard approach. Passing approximately 1200ft the AT controls the speed in MCP SPD, the AP flies in VOR/LOC and GS. Selected speed in Vref30+5, engine anti ice on. Underspeed (A) starts blinking in the speed window, no thrust change from the A/T. Instructor freezes the sim. Wants to know why the minium speed reversion is active. His answer: "Engine anti ice is on, the stall warning logic is set for icing. Our approach speed is too low". Riiiiight.

Yeah, use of TAI at present or WAI at any time during the flight will raise the minimum speeds to account for icing on the aircraft, but unless flying with flaps 15 (where Vref Ice exists for that purpose) that shouldn't be an issue? Was it maybe just the sim becoming desyncronized / the simulated FMC computing a wrong Vref? If this would be possible one should have seen it on the real aircraft, no? All sounds fishy to me.:}

[captplaystation]

13,000hrs on 737Classic/NG and I have never seen this, having no doubt made thousands of approaches at vRef30/40 plus5KT with TAI On.

Sounds like your instructor was trying to impress you with his supposed indepth knowledge, but in fact was talking a load of BS.

Unless I am missing something this last 22yrs. :rolleyes:

[Topper80]

I agree with you: Inever heard about Vref correction due to WAI ON during Flaps 30° or 40°. Only during a F 15 we use Vref ICE.

[shlittlenellie]

What Vref did you have set in the FMC? You'd get underspeed 'A' if you'd set Vref15.

[yrvld]

IF everything was how you described it and you didn't accidentally missed something, then you are right. There is no such thing as TAI speed correction for a normal F30 approach.
I would be curious to hear what kind of correction he wanted you to have and how you were supposed to calculate it.
The underspeed symbol might have appeared from some very simple reason, the A/T not keeping the speed right. What was your actual IAS at the time in relation to what you had set on the bug?

[STBYRUD]

I am absolutely positive, it was a standard F30 approach, Vref+5, TAI on, speed was exactly as commanded, wind was steady. The only F15 approaches we did during that session were single engine with A/T off. Never mind what was said in the debrief, this guy told us that it would be necessary to use a Vref Ice for all flap settings as a result of our observation - forget about it :ugh: He wasnt even convinced when confronted with the books, he seems to spend too much time in that sim (a shoddy Mechtronix)...

Aaaanyhow, just wanted to know if anyone by chance has witnessed similar quirks - thanks for your replies :)

[Denti]

Mechtronix, that is the problem right there. Doesn't use original parts and is therefore prone for mispresentation. Not a good training tool at all.

[STBYRUD]

Haha, I wholeheartedly agree :D Looks wrong, feels wrong - worst is the control feedback system that kicks in only long after the autopilot is disengaged... apart from the fact that apparently no single component is OEM in that box.

[stator vane]

i was brought to this thread by google when i typed in "stall warning logic"

there may be a point to uncover here.

in the Boeing Bible 737-800, in our version 3.20.7 it does say that the stall warning logic adjusts stickshaker and min maneuver speed bars on the IAS indication (tape) (we fly the 738's in STN that have a touch of blue)

it stays set when WAI is used in flight.

but there is a note that states that the FMC Vref speed is not adjusted!

so, perhaps, during the approach, if we use the FMC Vref speed plus 5 knots, then we might be slow enough to activate the underspeed blink.

the good book does not give us a clue as to what that adjustment is in knots that i can find. but i remember in the classics we used 10 knots to compensate for the fact that the stabilizer is not de-iced and there still might be ice on its leading edge reducing its effectiveness, hence the speed increase.

so, the instructor may actually be making a valid point, but the reduced books do not give us enough information to tell us how to compensate when the FMC Vref is not corrected.

there is that bit in the QRH in the single engine set up to change Vref 15.
that may be all we have at present!? that's 10 knots plus the 5 to fly. that matches what is in 4.20.21 (again-our version) Command speed limiting section.

and i have seen that underspeed many times! i suspect this is the cause.

chairs and rear guards.

[Denti]

There is no increment needed for Vref 30 or 40. Simply check your supplementary normal procedures, adverse weather, cold weather operation, approach and landing section. The only increment mentioned there is Vref ICE for flaps 15. In fact the first sentence is: Use normal procedures and reference speeds unless a flaps 15 landing is planned.

[Too Few Stripes]

From our supplementary procedures, ch16 adverse weather -

Approach and Landing
If ice formations are observed on the airplane surfaces, (wings,
windshield wipers, window frames, etc.):
VREF ............................................................ ..... Add 10 knots PF
This ensures manoeuvring capability.
Note: The combined airspeed corrections for ice formations, steady
wind and gust should not exceed a maximum of 20 knots.
Note: To prevent increased landing distance due to high airspeed,
bleed off airspeed in excess of VREF+5 knots+gust correction
when below 200 feet AGL. Maintain the gust correction to
touchdown.

[Denti]

Yup, thats the procedure for the classic (300-500). In the NG it is changed to

[737-700/800]
Use normal procedures and reference speeds unless a flaps 15 landing is planned.
If a flaps 15 landing will be made:
Set VREF 15
If any of the following conditions apply, set VREF ICE = VREF 15 + 10:
• engine anti–ice will be used during landing
• wing anti–ice has been used any time during the flight
• icing conditions were encountered during the flight and the landing temperature is below 10°C.

Since we do not operate the 900 our manuals only refer to 700/800, but i would think it is similar on the 900 as well.

[punk666]

You are correct, its exactly the same for the -900ER

[ImbracableCrunk]

The underspeed would be based on your current FMC weight and Vso, correct? If you manually input a weight was less than your current FMC weight, that could be a source of the error.

Not likely, but an idea. Not really an Ice issue.

[stator vane]

i simply quoted the book as it is.

i did not tell any one how to fly the airplane. that is up to the individual pilot and his company procedures. and his checker seemed to think there was a reason. i am certainly not one to tell the man with the ink pen that he is wrong unless i can show it in the books.

there must be a reason for the underspeed flashing.

the one who started the thread asked a question and i went to the books as they are and gave what might be a possible answer. perhaps others think they have the absolute truth.

it still stands that the FMC speeds are not updated by the stall logic.

and i have seen a few oversights in the books over the over 14,000 hours on the 737-1,2,3,4,5,800's i have flown and the various manuals attached.

[Denti]

Nothing wrong with quoting the book, but if you fly the NG and the classics you knows there are quite a few more differences than you see at first glance. Some of those differences seem to be about Vref speeds, so you need to add 10kts to all speeds on the classics and only to F15 on the NG.

The main reason for the SIM behaviour was the simulator in question. That particular mechtronix simulator (Class B) is a piece of junk, doesn't use official boeing information nor parts which accounts for the wrong indication probably, more so than a failure in boeings books on an airplane wich is now in service for 15 or 16 years already.

[stator vane]

with all due respect, STBYRUD asked the question and i was directing my response to him.

you sound quite authoritarian to tell me "you need to" ....

i have read the supplemental section.

i have seen several parts of the B737 manuals change since i've been reading them since 1992.

i don't fly with the misconception that Boeing is always right about everything.

back to STBY RUD,

i will speak with our Boeing rep and see what we can dig up about the stall warning logic, FMC not updating and exactly what triggers the underspeed flashing....i've seen it several times in the aircraft and let you know what we find. he has access to information not available to the masses.

[STBYRUD]

Thanks to all of you! Honestly I thought this thread was dead long ago, thanks especially to stator vane for checking with someone who can add something new to this! Honestly though I still blame the sim ;)

[Ditched]

Had excactly the same problem in the sim recently. Examiner stated it was just a sim fault and couldnt explain it...... it shouldnt happen at that stage.

regards