Crosswind Holding - Determining WCA (Wind Correction Angle)

[Jaybird180]

I would like some clarification on something regarding HOLDS.

Given the instruction: Hold at the XYZ VOR on the 180 radial, standard turns, I can visualize what the hold pattern will look like and I think it would be simple to execute, head to the VOR, make a standard rate turn (1 minute), head to the outbound, heading 180, standard rate turn and proceed for 1 minute back to the VOR fix. Simple, no fuss.

Throw in some wind - for example winds out of the west at 30kts and I loose understanding of something.

It is my understanding that hold headings tracks are to be uncorrected for wind (I have been watching the U of North Dakota youtube videos on HOLDS). If this is the case, the standard turn outbound from the fix will put the aircraft well downwind. How does the pilot know that he's been blown well clear (if this is the 1st outbound turn)? How many times should it take to determine the WCA to apply in the case where the outbound turn was made before any inbound leg has been executed to determine the WCA to remain on the 180 radial?

Edit: see above in bold

 

[bobmrg]

Your understanding is wrong. The wind-corrected outbound heading should be about (not much precision here) three times the inbound correction when you are tracking the radial inbound. See Figure 10-5 in the Instrument Flying Handbook.

Bob Gardner

 

[Jaybird180]

How do you determine the outboud WCA when you haven't yet flown the inbound?
Seems to me it would be tough just to get on the inbound.

 

[Jaybird180]

Your understanding is wrong. The wind-corrected outbound heading should be about (not much precision here) three times the inbound correction when you are tracking the radial inbound. See Figure 10-5 in the Instrument Flying Handbook.

Bob Gardner

And why would the outbound correction differ? If I need a 20* WCA on inbound, shouldn't I need the same 20* on outbound?

 

[jesse]

How do you determine the outboud WCA when you haven't yet flown the inbound?
Seems to me it would be tough just to get on the inbound.

You guess to a degree. Keep in mind that you're generally going to be doing one of the three hold entries and your exact position doesn't matter that much. Eventually though you'll intercept the inbound then you will have a clue what the wind is doing comparing what heading it is taking to track it. From there you make a guess at the outbound and you further refine that guess each time you go around.

It's not an exact science, just keeps you roughly in a chunk of airspace and out of a busy controllers hair.

 

[dmspilot]

And why would the outbound correction differ? If I need a 20* WCA on inbound, shouldn't I need the same 20* oNo n outbound?

No because you don't correct for wind during the turns (e.g. by varying the bank angle).

 

[John Collins]

Of course the inbound leg has positive course guidance and should be tracked. This will require using a wind correction angle after intercepting the inbound course. With a direct crosswind out of the west of 30 knots, the wind correction angle for a 90 Kt aircraft will be just under 20 degrees. No wind correction is used in the two turn segments, but the wind is blowing during them for roughly two minutes, during which time the aircraft will be displaced 1 NM from the no wind condition. If you only adjust the WCA for the downwind leg to the same 20 degrees, then you will still have drifted 1 NM from the inbound course. To compensate for this additional drift, one should double or even triple the inbound course correction (40 to 60 degrees) if you want to roll out near to being on the inbound course. If you happen to fly an aircraft that has air data and heading inputs to a GNS430W/530W or later, the holding pattern depiction on the map will be adjusted accordingly and not look like an elongated oval. If you have GPSS, it would attempt to fly the depicted course.

 

[poadeleted20]

How do you determine the outboud WCA when you haven't yet flown the inbound?

If you don't have a GPS, you estimate based on the forecast winds aloft, or the computed winds aloft if you've done that in flight.

Seems to me it would be tough just to get on the inbound.

Not that hard. Start your IR training and I'll teach you how. :wink2:

 

[moonshine]

Your understanding is wrong. The wind-corrected outbound heading should be about (not much precision here) three times the inbound correction when you are tracking the radial inbound. See Figure 10-5 in the Instrument Flying Handbook.

Bob Gardner

That was screwing me very, very bad on the Instrument checkride.
Say I needed 10 degrees of correction on the inbound. Fine, so I give it 30 on the outbound. I turn inbount.. radial is nowhere to be found. DPE must have been laughing on the inside
30some knots quartering tailwind from the right (as seen on the inbound) was at just the right angle (say 20 degrees off), so when I turned outbound with 30 degree correction, istead of countering the wind, I was getting blown further off course with the wind some 10 degrees off my right again.
Fun. Took me two rounds to figure it out, third run was good and I got to move on with my life

 

[bobmrg]

And why would the outbound correction differ? If I need a 20* WCA on inbound, shouldn't I need the same 20* on outbound?

You guess at the outbound...too much is better than not enough. I'll defer to John Collins for the rest.

Read the FAA publications, that's what they are for, and it is the reason why they are listed as references in the PTS.

Bob

 

[Jaybird180]

Of course the inbound leg has positive course guidance and should be tracked. This will require using a wind correction angle after intercepting the inbound course. With a direct crosswind out of the west of 30 knots, the wind correction angle for a 90 Kt aircraft will be just under 20 degrees. No wind correction is used in the two turn segments, but the wind is blowing during them for roughly two minutes, during which time the aircraft will be displaced 1 NM from the no wind condition. If you only adjust the WCA for the downwind leg to the same 20 degrees, then you will still have drifted 1 NM from the inbound course. To compensate for this additional drift, one should double or even triple the inbound course correction (40 to 60 degrees) if you want to roll out near to being on the inbound course. If you happen to fly an aircraft that has air data and heading inputs to a GNS430W/530W or later, the holding pattern depiction on the map will be adjusted accordingly and not look like an elongated oval. If you have GPSS, it would attempt to fly the depicted course.

It's not an exact science, just keeps you roughly in a chunk of airspace and out of a busy controllers hair.

If you don't have a GPS, you estimate based on the forecast winds aloft, or the computed winds aloft if you've done that in flight.

Makes more sense now. I guess I thought it was a precision manuever.

Start your IR training and I'll teach you how. :wink2:

rgr

 

[poadeleted20]

Makes more sense now. I guess I thought it was a precision manuever.

Holding pattern a precision maneuver? Now you know why the FAA makes such a big protected space around them. :wink2:

 

[Jaybird180]

Haven't read the answer in any regulation yet to be sure, but am I assured of at least vertical precision from other pilots in the theoretical 1,000 foot separation stack?

 

[poadeleted20]

Haven't read the answer in any regulation yet to be sure, but am I assured of at least vertical precision from other pilots in the theoretical 1,000 foot separation stack?

ATC doesn't get excited until you go more than 200 feet off. Essentially, that really guarantees you only about 500 feet of separation (assuming you're both off altitude towards each other). That's why the FAA requires 1000 feet vertical separation under IFR -- 200+ feet of altitude reporting error, up to 75 feet of preflight altimeter error, etc, etc. To misquote the late, great Senator Everett McKinley Dirksen (R-IL), "A couple of feet here, a couple of feet there, and after a while, you're talking about a lot of altitude."

 

[gismo]

That was screwing me very, very bad on the Instrument checkride.
Say I needed 10 degrees of correction on the inbound. Fine, so I give it 30 on the outbound. I turn inbount.. radial is nowhere to be found. DPE must have been laughing on the inside
30some knots quartering tailwind from the right (as seen on the inbound) was at just the right angle (say 20 degrees off), so when I turned outbound with 30 degree correction, istead of countering the wind, I was getting blown further off course with the wind some 10 degrees off my right again.
Fun. Took me two rounds to figure it out, third run was good and I got to move on with my life

Several years ago I put together a simulation of holding in wind and found that the angle of the wind relative to the inbound course has as much or more effect than the wind velocity on the necessary outbound heading. One thing I learned is that (assuming one minute inbound legs, standard rate turns, and two seconds to roll into and out of the turns) there is no fixed relationship between the inbound wind correction angle and the outbound one. Depending on the wind strength and direction that ratio ranged from 2.5 to almost 9 although the largest deviations from 3 were when both wind correction angles were small.

Of more concern is how much error there is in the recommended 3x inbound correction for the outbound leg. This is a very nonlinear function of the wind strength with worst case errors of about 5° with a wind velocity equal to 10% of the holding TAS, 9° at 15% 18° at 20%, 33° at 25%, and a whoppint 65° at 30%. Interestingly when the wind is a little more than 30% of the TAS, it's simply not possible to fly a hold with standard rate turns and one minute inbound legs with some wind directions.

With a 90 Kt holding speed it doesn't take but 20 knots of wind from 45° into the holding side (quartering tailwind on the outbound leg) to require more than 45 degrees of wind correction away from the inbound. That's more than 5 times the inbound WCA of 9 degrees.

If you lengthen the inbound leg things get a lot better.

 

[moonshine]

Figured I'd make this visual

Here's three holds - two from practice runs, third one from the checkride

First one - busted the timing for the entry outbound, otherwise ok
Second - entry overflight was "mm.. close nuf" (IIRC this was one of the earlier tries at the whole hold business)
Third one.. u-g-l-y. 30kts winds out of NW. Hey, stayed in the protected area and passed the ride, which was one of those where unusual attitudes were incorporated into the flight by Mother Nature

 

[EdFred]

Several years ago I put together a simulation of holding in wind and found that the angle of the wind relative to the inbound course has as much or more effect than the wind velocity on the necessary outbound heading. One thing I learned is that (assuming one minute inbound legs, standard rate turns, and two seconds to roll into and out of the turns) there is no fixed relationship between the inbound wind correction angle and the outbound one. Depending on the wind strength and direction that ratio ranged from 2.5 to almost 9 although the largest deviations from 3 were when both wind correction angles were small.

Of more concern is how much error there is in the recommended 3x inbound correction for the outbound leg. This is a very nonlinear function of the wind strength with worst case errors of about 5° with a wind velocity equal to 10% of the holding TAS, 9° at 15% 18° at 20%, 33° at 25%, and a whoppint 65° at 30%. Interestingly when the wind is a little more than 30% of the TAS, it's simply not possible to fly a hold with standard rate turns and one minute inbound legs with some wind directions.

With a 90 Kt holding speed it doesn't take but 20 knots of wind from 45° into the holding side (quartering tailwind on the outbound leg) to require more than 45 degrees of wind correction away from the inbound. That's more than 5 times the inbound WCA of 9 degrees.

If you lengthen the inbound leg things get a lot better.

On my checkride it was impossible to get a 1 minute inbound leg. 0° WCA because it was straight on the nose on the inbound, but after it took 2 minutes to get back to the fix, with a 30 second outbound leg, I turned to the DE and said, 1 minute is impossible to do on this, even if I do a constant 360° turn. He says lets find out, and it was 90 seconds inbound.

 

[DutchessFlier]

First time I flew with the GPSS installed with the 430W and let it fly a hold with a pretty good crosswind component, I was pretty amazed at how accurate it was flying the depicted course. Had a hard time not trying to fly it by hand, and truthfully it was an uncomfortable sensation not controlling the plane by hand and just letting it fly using the autopilot.

Now, for working in the airspace around here, I don't fly the autopilot all the time, but when it's busy and I'm alone in the crap, it's on.