737 Vref Ice

[varkdriver495]

Any ideas why VREF ICE correction is only applied to the Flaps 15 landing configuration? I can't see why a PLUS 10 knot additive would not be advisable for all landing flap configurations if icing was encounter or planned for...any thoughts.

[Rainboe]

That's only for the NG models 600-900. The Classic 300-500 has a different criteria (what does one call the 100-200 now? Dinosaur?).

That is how Boeing determined it. As pilots, we follow the guidelines that come from the manufacturer. It has been decided we don't have to know the reason, just do it! I guess the reason is that it has been determined that the NG wing, which is markedly different to previous versions, is not significantly adversely affected by icing in normal landing configurations.

[parkfell]

And not to forget that flap 15 is for OEI landings; unless the TE flaps etc are an issue.

[Mad (Flt) Scientist]

... the NG wing, which is markedly different to previous versions, is not significantly adversely affected by icing in normal landing configurations.

Or that it is affected, but the effect is already accounted for in the numbers used for all cases, ice and no ice, and therefore no "delta" for ice is required.

[varkdriver495]

That's my thought...is that margins at flaps 15 are tight, so 10 knots are added for icing conditions...but not necessary for 30/40 flap landings...

[Rainboe]

So why would Boeing licence the aeroplane with tighter margins at Flap 15 than for greater settings? Is it an admission that icing affects a partially flapped wing more than a fully flapped wing?

[Mad (Flt) Scientist]

This is speculation without seeing the actual design info, but ....

There are generally three sets of icing shapes considered in certifying an aircraft - the fully developed ice shapes (which exist only on unprotected parts of the airframe), the "failure" ice shapes, representing the worst case ice accumulation on the protected parts of the airframe following a failure, and the "encounter" or "delayed turn on" (DTO) shapes, representing the ice that will accumulated on an unexpected encounter with ice, before the system is turned on.

Since the conditions being discussed are not failure cases of the icing system, we only need be concerned with the unprotected ice (often called 3 inch ice, since that's the standard height assumed by most) and the DTO ice.

3 inch ice will be on all the unprotected surfaces when in icing conditions; DTO has to be assumed even for "no ice" conditions.

IF the location of the unprotected area is such that it affects the stall onset only at flaps 15 then that would explain the reason for an adder only for that case. It is possible that it actually affects all the configurations relative to a clean wing, but that landing flaps are sufficiently affected by DTO - which is considered for all conditions - that the 3 inch effect is masked.

Since flaps can cause the stall to initiate at different spanwise locations, this is a possible explanation.

[FE Hoppy]

Could the answer be that in the no addition configurations the Vref is already well above Vsl and is in-fact increased to give correct landing climb gradient?

I ask because I think this is the case on the E-jets with flaps full.

[Chesty Morgan]

Hoppy the speeds used on the 190/195, and I think the 170/175, all ready include an icing increment for both landing flap configurations (5 and Full).

In effect we fly around using icing approach speeds whether we're in icing or not. Vref for both settings is the same as Vac (approach climb speed).

[FE Hoppy]

Not quite Chesty.

On your 195s they are all the same but on the 190s and 70/75s they're not. And FAA certified jets are different again. Plus you also have an option for Alternate go around flap on JAA certified jets which gives a whole different set of figures just as CAT ll/lll does.

It's often as not about the approach climb/landing climb requirements rather than the margin above Vsl on these jets.

[Checkboard]

That's my thought...is that margins at flaps 15 are tight, so 10 knots are added for icing conditions...but not necessary for 30/40 flap landings...

That's the reason. When any aircraft is flying with flap set, it is happy at any speed between the maximum flap operating speed, and a margin above the stall (usually 30% - i.e. 1.3 Vs). Boeing decided to "simplify" this philosophy by recommending manoeuvring speeds, set roughly half way between the two limits, but also adjusted to make them easy to remember and a "logical" progression as you select the different flap stages. (Other aircraft don't have single set speeds, but rely on a bit of pilotage :hmm: )

The manoeuvring margins are shown in graphical form in the [edit: the flight crew training manual] - and it is here that you can see the flap 15 manoeuvring speed is a bit closer to the low end of the range than the other recommended flap speeds. On the chance that ice has formed on the parts of the airframe not protected by de-ice, increasing the weight (and thus stall speed) the flap 15 manoeuvring margin is the only one that is close enough to the bottom edge of the speed range to worry about increasing it.

[FE Hoppy]

Checkboard:

We are talking Vref here so Boeing most definitely didn't just pick a number in the middle.

[Chesty Morgan]

Hoppy, roger. Should maybe know that seeing as the type rating covers 170-195. oh well:}

[FE Hoppy]

Chesty:
Shhhhh!! you'll get me in trouble. They'll be asking for an extra days perf course to cover the diffs.

[six string]

B737-800. All other flap 15 events direct to vref ice, except this one.

[Australopithecus]

B737-800. All other flap 15 events direct to vref ice, except this one.

Because the checklist then directs you to set Vref 15 + 15, unlike the other non-normals requiring flaps 15

[six string]

good catch!