Situations for reduced flaps

[Bernoulli]

I am interested in reasons why pilots would choose to use less than full flaps during an approach to landing especially in regards to cross wind landings, wind shear, and high density altitude. Any comments would be greatly appreciated. Thanks in advance for any comments.

 

[mar]

A good question.

Usually when a pilots elects to land with less than full flaps it's because a higher than normal approach speed is desired--and hopefully runway length and contamination won't be an issue (if it is, then you need to weigh the pros and cons).

When are higher than normal approach speeds desirable?

--When you need more protection above the stall in gusty conditions (wind shear).

--When you need more controllability in a stiff crosswind.

--When you're iced up pretty good and you not only want more protection above the stall but you also don't want to change the downwash over the horizontal stab by lowering flaps.

With regard to the high density altitude airport operation, every thing being equal, there shouldn't be any change to your profile. You would fly the same configuration and same IAS. The only difference will be your TAS and GS on touchdown. Obviously you're gonna use more distance.

I wouldn't adjust my flap setting on approach to a high density altitude airport unless things were gusty and/or icy.

Full flaps are nice for flying slow approaches but sometimes you just don't want to be so 'draggy'.

I hope it makes sense. If not I'm sure someone will help clarify it.

 

[mckpickle]

When using less flaps the A/C tends to resond quicker. IE better controlability. A 172 with 40 degrees is a bit more slugish and will float over the ground more at 40 than landing at 20. You retain the aileron and rudder input by maintaining a higher airspeed while also lowering your stall speed.

In the ERJ when landing in heavy crosswinds and with plenty of runway, we might use flaps 22 instead of flaps 45. The aproach is flown faster but tends to be more controlable and stay on the ground once there.

 

[bobbysamd]

It is a good question . . . .

In a high-wing aircraft, the flaps reduce the airflow over the vertical stabilizer and rudder, making them less effective. I always had better luck with crosswind landings in 172s when I backed off on flaps.

It depends on the crosswind. If it is only a few knots, backing off on flaps may not help appreciably. But, with five or more knots of crosswind, you'll see the difference.

You may not want to use full flaps during an approach in gusty winds and/or turbulence. Backing off on flaps will improve controllability. You want to carry more speed into the approach - perhaps half to all of the gust factor - and fly a powered approach all the way down. Less flaps with more power and speed will, again, improve controllability. With reported shear, you definitely want to carry more speed.

If you have doubts that less flaps and/or more speed will cause you problems getting in on your runway, look at your landing chart. See how headwind component shortens your landing distance.

Hope that helps a little more.

 

[Bernoulli]

Thanks Mar and mckpickle for responding.

Mar...Regarding high density altitude... don't many pilots tend to make their approach with less than normal flaps in case a go-around is needed? I think I have read that before but I can't find it now. Something to the effect that when full power is applied for a go-around the flaps and gear will likely prohibit a plane from climbing in a high density situation.

mckpickle... regarding using less flaps. I can see why you would maintain more airspeed which helps the rudder and aileron control but with less flaps doesn't the stall speed increase rather than decrease? I though flaps decreases the stall speed.

 

[mar]

Go arounds at high density altitude

Bernoulli--I see what you're saying now. I suppose that if you're flying a non-turbocharged engine (normally aspirated) then you might already consider yourself "behind the power curve" so to speak.

But unless you had some sort of malfunction (electrical, hydraulic...) the gear and flaps are supposed to come up! So I wouldn't start changing the program too much just because the air's pretty thin. But certainly a go-around at high density altitudes requires some planning.

But these are all things you should be considering anyway and there's a name for it: 'Airmanship'. There are lots of different techniques out there. When you start flying commerically the company will try to train out most technique-flying and enforce 'standardization'...

...but there's always room for technique. I think a good pilot can differentiate between *procedure* (e.g. Lower the gear at FAF inbound) and *technique* (turn on the taxi light when cleared to land).

Airmanship develops from a combination of skill, judgement, experience and discipline.

I'm off on a tangent now but I'm basically trying to say "Fly the plane the way it was designed--don't try to reinvent the wheel--but stay flexible enough that you can change your routine to adapt to different situations."

Good luck.

 

[mckpickle]

Sorry I should have been more specific. Some people will choose to fly a no-flap approach in say a 172, in heavy wind. Landing with 10-20 will reduce your stall speed. Landing with 20 will also reduce your stall speed without increasing your drag to what 40 would do. Sorry.

 

[Bernoulli]

Mar stated: When you're iced up pretty good and you not only want more protection above the stall but you also don't want to change the downwash over the horizontal stab by lowering flaps.

Do most of you guys who fly in icing come in with no flaps when there is ice on the wings? What is the rule... partial flaps no flaps, full flaps? I have never flown in icing and don't know enough about flap techniques in icing conditions. Can any one elaborate on this?... especially how flaps would change the downwash over the horizontal stab and why that is critical when ice has formed. Thanks in advance for the info.

 

[mar]

I wish I had a chalkboard

First, there are no rules for flap operation in icing conditions. But as a general rule of thumb, if you're in an airplane with no ice protection and you've packed on a lot of ice (you're pulling a lot of power to just maintain airspeed) then you're better off making the approach with the minimum flap setting you can handle given the length of the runway.

Why?

First you need to understand the wing produces a downwash. You might see evidence of it by the stains on the fuselage. Second you need to understand the horiz stab produces "Lift" in a negative direction, in other words, it produces a "tail down force."

The reason the tail produces that negative lift is to provide stability--and that can be a whole topic all by itself so I'll just let that stand for now.

When you lower flaps under any circumstance you change the airflow (increase the downwash) over the horiz stab. Essentially you're increasing the angle of attack (in a negative direction) of the horiz stab.

Now...if you're iced up you're flying a wing and a horiz stab with a *new* camber shape and guess what? You don't know what the new critical angle of attack is of that airfoil.

You don't know the new stall speeds. Thus, on approach you're much safer approaching at a higher airspeed (to protect the wing against stalling) and not lowering the flaps to full (increasing the downwash and the critical angle of attack on the horiz stab).

The accident databases are full of stories about airplanes that iced up, flew a successful approach only to lower the flaps, stall out the tail, pitch down and crash short of the runway.

If you're compelled to lower the flaps in such a situation be modest. Maybe 10 or 15 degrees but definitely not full (30 to 40 degrees).

I hope that makes sense

 

[Bernoulli]

Thanks Mar for the explanation. Makes total sense. I appreciate the info.

 

[sky37d]

Re: I wish I had a chalkboard

First, there are no rules for flap operation in icing conditions.

You don't know the new stall speeds.

I hope that makes sense

At this point in time, you are now a test pilot, on a plane for which there is no documentation. Ice can be a scary thing. Read Duke's tale about flying in ice. I have done it, had 4 inches on the front of the radome when I landed. 1 notch of flaps, no more.

It is my theory, and again anyone who tries this becomes a test pilot, is that when you have ice, and you extend the flaps 1 notch, you are helping to break it up. My A/C has de-ice boots, but I had so much ice, it really affected the way the plane flew.