LLWS can be defeated

Jaybird180

Jaybird180

Final Approach
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Jaybird180
denverpilot said:
The critical part is how high AGL it's all going on. That's what was causing all the LLWS on Saturday last when Doug and I were up. 15 knot loss of airspeed at 300' AGL.
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This post got me thinking (again) about what stall angle of attack means and it's relationship to airspeed. In a landing situation if the aircraft is flying a stabilized approach at 1.3VSo and LLWS at 300' AGL reduces the airspeed below VSo, will the aircraft stall? Has the pitch angle changed?

Since we know that stall speed reduces with weight, we can ascertain that the wing will continue to produce sufficient lift at any airspeed at or above the stall speed, but there have been discussions ad-infinitum explaining how Indicated Airspeed is a poor metric for predicting when an aircraft will stall.

Assuming it could be simulated in a lab environment to exacting measurements, could the aircraft be unloaded by .0001G to reduce lift requirements thereby avoiding the loss of lift in this situation?
 
What happens in that situation is the airspeed drops, and with it lift (which is proportional to V-squared), and the airplane then drops without changing pitch attitude. That increases AoA, possibly to the stall point. That's why the first thing to do in a LLWS airspeed loss is add power to increase excess power and prevent the airplane from dropping before the airspeed recovers. The good news in light singles is their sectional density is so low and engine response so fast, about all you see is a momentary blip in airspeed before recovery. It's the big heavy jets with high sectional density and long spool-up times which have the big problems.
 
Ron Levy said:
What happens in that situation is the airspeed drops, and with it lift (which is proportional to V-squared), and the airplane then drops without changing pitch attitude. That increases AoA, possibly to the stall point. That's why the first thing to do in a LLWS airspeed loss is add power to increase excess power and prevent the airplane from dropping before the airspeed recovers. The good news in light singles is their sectional density is so low and engine response so fast, about all you see is a momentary blip in airspeed before recovery. It's the big heavy jets with high sectional density and long spool-up times which have the big problems.
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Damn you Ron Levy! (being so lucid).:rofl:
 
sure you could unload to below 1G to prevent the stall, but in return you would be descending faster when close to the ground. Sometimes that is your only choice though.
 
Jaybird180 said:
Since we know that stall speed reduces with weight, we can ascertain that the wing will continue to produce sufficient lift at any airspeed at or above the stall speed, but there have been discussions ad-infinitum explaining how Indicated Airspeed is a poor metric for predicting when an aircraft will stall.
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OK, this kinda bugs me. While I agree that the book numbers for stall speed may not agree with the airspeed indicator, the ASI is still self consistent. I mean that if you go up, fly to a stall, and check the ASI, then whatever it reads is the indicated stall speed for that aircraft configuration and atmospheric condition. So if you find that spot on the ASI on Monday and go fly again in identical conditions on Thursday the plane will stall at the same point on the ASI.
 
COFlyBoy said:
OK, this kinda bugs me. While I agree that the book numbers for stall speed may not agree with the airspeed indicator, the ASI is still self consistent. I mean that if you go up, fly to a stall, and check the ASI, then whatever it reads is the indicated stall speed for that aircraft configuration and atmospheric condition. So if you find that spot on the ASI on Monday and go fly again in identical conditions on Thursday the plane will stall at the same point on the ASI.
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You're absolutely right.
But that's moot.

The problem isn't that the airspeed indicator will read the same speed at the stall when the airplane's in the exact same aerodynamic situation on Monday and Tuesday.

The problem is that the airplane will stall at different airspeeds based on different aerodynamic situations. Turning? Different airspeed at the stall than level. Contaminated wings? Different airspeed at the stall than clean wings. So unless you keep a table of the various "situations" and the stall speed, like a test pilot, it's not very useful.

By contrast, an AOA indicator is much more consistent and agnostic of the aerodynamic situation.
 
To illustrate Tims point I've been in a B727 at 40 kts before. We were not stalled.
 
TMetzinger said:
The problem isn't that the airspeed indicator will read the same speed at the stall when the airplane's in the exact same aerodynamic situation on Monday and Tuesday.

The problem is that the airplane will stall at different airspeeds based on different aerodynamic situations. Turning? Different airspeed at the stall than level. Contaminated wings? Different airspeed at the stall than clean wings. So unless you keep a table of the various "situations" and the stall speed, like a test pilot, it's not very useful.

By contrast, an AOA indicator is much more consistent and agnostic of the aerodynamic situation.
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The wing may stall at a different AoA if it's contaminated, since the effective shape of the airfoil is changed. The AoA indicator is still *less* sensitive to the aerodynamic situation than an ASI, so it's a Good Thing to have.
 
Jaybird180 said:
This post got me thinking (again) about what stall angle of attack means and it's relationship to airspeed. In a landing situation if the aircraft is flying a stabilized approach at 1.3VSo and LLWS at 300' AGL reduces the airspeed below VSo, will the aircraft stall? Has the pitch angle changed?
Click to expand...

Yes lol, happened to me at 3' :D. The pitch doesn't change as in a stall break, you just sink for a moment until gravity and the wind overcome your inertial lag and you start flying again. Unfortunately 3' is not enough altitude for this to happen. 10' and I would have been ok.
 
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Ron Levy said:
What happens in that situation is the airspeed drops, and with it lift (which is proportional to V-squared), and the airplane then drops without changing pitch attitude. That increases AoA, possibly to the stall point. That's why the first thing to do in a LLWS airspeed loss is add power to increase excess power and prevent the airplane from dropping before the airspeed recovers. The good news in light singles is their sectional density is so low and engine response so fast, about all you see is a momentary blip in airspeed before recovery. It's the big heavy jets with high sectional density and long spool-up times which have the big problems.
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A nice explanation.

As Ron says, big jets with long spool times are in trouble in those conditions. Recovery is pretty quick in a light piston single but not instantaneous.

No stopwatch handy, obviously, but Doug did the correct recovery (throttle up, and definitely don't pull up! -- he didn't have to push the nose down, but we had a little altitude left to trade if needed), I'd guess I counted about 2-3 seconds at 45 knots indicated and a blip of the stall warning. We still had some margin, but not much.

(Stall warning comes in well before the stall in a Skyhawk if you're not pitching heavily, and the difference between calibrated and indicated airspeed increases in a Cessna at low speeds, both indicated that we were still flying... plus the floor wasn't dropping out by "calibrated butt feel".)

Once it recovers you have to be ready to get the throttle back out or it's going to accelerate above your target speed (approach in this case) with all that extra power in, and it'll do that as quickly as you're used to seeing under normal conditions.

Doug was originally flying it about 65 and slightly slowing, when I said "fifty-five, FOURTY-FIVE" about as fast as you can say that phrase at a non-panic'ed speed, to give a feel for how quick it occurs.

I'd been chanting sixty-five for about three repeats, after he was stabilized -- and was about to say "sixty" when I said the other two, back to back.

At the 55 call he shoved power in. The power stopped the trend (movement) downward of the ASI at 45 and within a couple of seconds it was trending the other direction and he then put the power back where it was, I'm sure just by ear and feel. (He didn't have time to look inside, I called numbers, he landed in the crosswind.)

He later said he was on a go-around hair trigger and the landing was one of those where, "I wasn't going to land... until I did." ;)

Without someone calling out airspeeds so he could have stayed visual (for the crosswind), he said he would have gone around and asked for the crosswind runway.
 
Re jets and the spool up time. Spool up applies primarily to going from idle to power. Jet pilots are always stabilised by 500 VFR and 1,000 IFR minimum. Stabilised means engines spooled up and on speed. Also, the mass of a jet counters much of the fall from lost lift in a gust so it's easier to counter.

Also, every plane I've flown over 12,500 has added speed to the ref for just that reason.Usually its half the steady wind plhalt he gust with a max of 20 kts.

I'm not saying what anybody has said here is wrong, I'm just adding to it all.
 
Captain said:
Re jets and the spool up time. Spool up applies primarily to going from idle to power. Jet pilots are always stabilised by 500 VFR and 1,000 IFR minimum. Stabilised means engines spooled up and on speed. Also, the mass of a jet counters much of the fall from lost lift in a gust so it's easier to counter.
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First, how spooled up you are depends on what you're flying. Second, it's that high mass/presented area ratio which makes the effects worse on big, heavy jets.

Also, every plane I've flown over 12,500 has added speed to the ref for just that reason.Usually its half the steady wind plhalt he gust with a max of 20 kts.
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For light planes, we usually teach adding half the gust factor computed as follows:

GF = peak gust - steady state

So, if the wind is 12G20, the GF is 20-12=8, half the GF is 4, so add 4 knots to normal final approach speed.
 
Unless you're flying some really old straight pipe turbojet, modern turbofan engines will spool up considerably quicker from idle.

Also, as mentioned with jet operations you are configured (gear and flaps) on approach so with the drag out the engines are already spooled and will react quickly to a TOGA application of power.
 
Ron Levy said:
First, how spooled up you are depends on what you're flying. Second, it's that high mass/presented area ratio which makes the effects worse on big, heavy jets.

For light planes, we usually teach adding half the gust factor computed as follows:

GF = peak gust - steady state

So, if the wind is 12G20, the GF is 20-12=8, half the GF is 4, so add 4 knots to normal final approach speed.
Click to expand...

Just as a reference to your post:

My airline is 1/2 steady state plus gust added to Vref up to +20. So in that example it would be 6+8=14kts added to Vref. I would assume the different formulas to calculate is directly related to the inertia issue like you mentioned.



I know it's not quite what we're talking about here, but I've flown through 3 LLWS microburst events. When I first read the title of the thread I came in to scream "NO NO NO DON'T EVEN THINK IT." It will scare the ever-loving sh*t out of you. Nothing like full power with 10-15 degrees nose up and seeing your VSI showing 2000fpm.. down.
 
Rex Kramer said:
Just as a reference to your post:

My airline is 1/2 steady state plus gust added to Vref up to +20. So in that example it would be 6+8=14kts added to Vref. I would assume the different formulas to calculate is directly related to the inertia issue like you mentioned.
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It is indeed.

I know it's not quite what we're talking about here, but I've flown through 3 LLWS microburst events. When I first read the title of the thread I came in to scream "NO NO NO DON'T EVEN THINK IT." It will scare the ever-loving sh*t out of you. Nothing like full power with 10-15 degrees nose up and seeing your VSI showing 2000fpm.. down.
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Yup -- that's what flying those big heavy airplanes will get you. Fly a light single like most folks hear and you're just a leaf. For us, the big threat is the downburst with more velocity down than we can generate up, not the shift in horizontal wind velocity affecting airspeed.
 
Rotor&Wing said:
Unless you're flying some really old straight pipe turbojet, modern turbofan engines will spool up considerably quicker from idle.
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Well, my initial fighter-type experience was a centrifugal-flow P&W J48 with a 13 second idle-to-mil time in the TF-9J Cougar. Also, later on, 900 hours in the F-111 on the TF30, the earliest afterburning turbofan (IIRC), which wasn't the world's fastest from idle to mil even with only a 1:1 bypass ratio. The slow spool-up on that almost caused me test the ejection system one late spring afternoon in England after a compressor stall on the "good" engine on the go after a simulated single-engine approach. One engine stalled, one engine at idle, flaps 2/3, gear in transition (which meant speed brake extended) -- not a happy situation at 100 AGL with only 160 KIAS. We bottomed out at 142 KIAS and about 40 AGL before the other engine spooled up and the sink stopped. :hairraise: I didn't unwrap my hand from the ejection handle until we had about 180 KIAS and the unstalled engine at mil power.

And yeah, there was something in the Dash-1 section on go-around out of a simulated single-engine approach about not plugging the burner on the "good" engine until the other engine had spooled up, but my buddy in the left seat forgot that, and I failed to prevent him from doing that foolish thing.:redface:

So, consider the Law of Primacy when understanding why low RPM on a turbine engine near the ground gets me tensed up.

OTOH, the straight axial flow turbojet J52-P8 in the A-6 Intruder had great response from low power (just not enough max thrust for the aircraft's weight, I thought).

Also, as mentioned with jet operations you are configured (gear and flaps) on approach so with the drag out the engines are already spooled and will react quickly to a TOGA application of power.
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That does help, but again, it depends what you're flying. They had to modify the original S-3A Viking design with spoilers for direct lift control on final to keep the relatively high bypass (for a carrier jet) fan engines spooled enough for a wave-off or bolter because even with flaps/gear/boards, pulling the power enough to correct for high on the ball would put the engine too low to spool quickly -- something not discovered until the initial carrier suitability trials. :eek:
 
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