I was just watching some jets leave their contrails across the sky, and I realised that the high altitude winds must sometimes be quite swift.
One contrail was smeared out perpedicular to the plane’s line of movement withing moments. Meaning some very heavy crosswinds.
Remembering all those times in light planes with the pilot crabbing (turning the nose of the plane slightly into the wind while maintaining the same heading), I wondered, do those 500 to 600 mph jets at 35,000 to 50,000 ft ever have to crab?
The winds get a lot stronger as you get higher. If you are higher enough you can get into the jet stream.
In any wind, an aircraft’s heading isn’t going to be the same as its track across the ground unless it is a direct headwind or tailwind. The difference can be any thing from 1 degree to over 20 degrees. The difference between heading and track is called drift, an aircraft has a wind correction angle (WCA) to counter the drift and attempt to align the aircraft’s track with the intended course that you wish to take. A faster aircraft will require less WCA to counter a cross wind. However most fast aircraft fly higher than slower aircraft (excepting gliders) so the winds are stronger and they may require significant WCA.
As an example, an aircraft doing 500kts experiencing a cross wind of 50kts would require a wind correction angle of 6 degrees. So if our aircraft wanted to track south (180) and the wind was coming from the west (270) then it would have to have a heading of 186 to maintain its desired southerly course.
If a slower aircraft travelling at, say, 120kts encountered the same 50 kt crosswind it’s heading would have to be 205. A full 25 degrees off its intended course. Also, because the aircraft is now pointed significantly into wind to counter the drift, it is experiencing a slight headwind in addition to the crosswind and it’s speed would be slowed to 109 kts
The effect as you’ve seen it will be more pronounced with stronger winds. Also the contrail will continue to move laterally with the wind and may make the effect appear stronger.
Another comment: A 50 knot wind is absolutely ordinary at any decently high altitude. Jet stream winds are, by definition, a minimum of 50 knots. It is common to see much faster winds than that.
So an airliner flying almost 500 knots with a 100 knot crosswind, requires the same amount of crosswind correction as a Cessna flying at 100 knots with a 20 knot crosswind.
Here’s a map of the current jet stream around the souther hemisphere, The lighter shades inside the jet stream indicate stronger. Right now there are winds up to around 130 kts in this region. You can see that if you were to fly direct from Sydney or Brisbane (about 1/3 to 1/2 the way up the east coast of Australia) to the south of New Zealand, you’d be affected by quite strong crosswinds. If you went to the north of NZ you’d get into the following jetstream with little crosswind, you’d have a high ground speed. This accounts for significant differences in travel time going in one direction as opposed to the other.
Oklahoma City, aircraft traveling almost due West, wind apparantly from SSE smearing the contrail almost instantly (looked like cirrus clouds within a few minutes). I didn’t think the Jet Stream was right over us right now. Could it be a typical Spring pattern inflow, or is that usually limited to the Troposphere?
Oklahoma currently has the jet stream directly overhead at around 100 kts from the west-southwest. Further to the NW the strength is over 110 kts.
The contrail getting smeared instantly is not necessarily due to the aircraft “crabbing” it’s more likely that a strong slightly turbulent wind is quickly disturbing and dispersing the contrail.
Okay, I often get directions mixed up (plus, I may not have seen it traveling due West), so the Jet Stream might be causing the cirrus pattern I saw. And a turbulance causing the immediate smearing makes sense.
Just to add to the terrific description from 1920’s Style Death Ray:
With a crosswind, if you were to point the nose of the airplane at your destination and keep it pointed at the destination, you would find that your heading would be constantly changing, and your course over the ground would be an arc. This is actually common with pilots improperly flying ADF (radio direction finding) instruments. An ADF only tells you the heading to a station - it does not descibe your track on the ground. There are ways to use your ADF to calculate the crosswind and therefore the amount of crab angle you need into the wind to fly a direct course to the beacon, but I’ve seen a surprising number of pilots screw that up or ignore it.
In contrast, a GPS or a VOR tell you your actual track over the ground, and you can fairly easily adjust your crab angle to maintain a straight line course to your destination.
LOL, You are assuming that someone taught them what ADF is, and how to fly it and that they are capable of flying straight and level and maintaining a heading for any length of time.
Autopilot will do it for them I suppose… ( straight and level )
Any pilots here under 50 years old that have actually flown using ADF as primary navigation? Had over an hour practice total, using it in their entire flying career ? Not counting watching it switch as an old fashioned ‘marker’ beacon in some of the older aircraft where they did not have all the latest goodies?
Not saying it is not there and turned on just like the book says it should be, but do they even look at it after their instrument check ride?
Yeah, I’m old and it scares me the number of licensed commercial and instrument rated pilots that can not get from A to B with just a Sectional map.
I’m 40, and I learned to fly an ADF. It helped that we were flying out of a military base that had a ‘DF Steer’ precision approach along with the nonprecision ADF approach. It was great asking for a Precision Radar Approach. The controllers liked the practice.
In case you’ve never flown one, a PAR approach is flown by simply following the instructions of a ground controller who is staring at a very accurate radar scope. The approach sounds something like this (if I can remember after 15 years…)
PAR Approach: “Yankee Hotel Foxtrot, all turns rate one. Turn left, heading 135.”
YHF: “Turning 135.”
PAR: “Yankee Hotel Foxtrot, on course, on altitude…No further communication from you required. Yankee Hotel Foxtrot, turn left…now. Turn right… NOW. Slightly under glidepath, adjust your rate of descent. Turn left…now. Level wings…now. All turns half rate. Turn right, now. Slightly above glidepath, adjust your rate of descent. On course, on altitude. Contact tower 118.0”
Etc. They could put you right on the button of the runway every time.
Back to the ADF: When I bought my plane, that was the first new instrument I put in it. I found flying ADF fun, and when you wanted to relax you had that hi-fi AM radio to listen to. But this is in Canada, and an ADF was always more useful up here than in the States, where youv’e got VORs dotting the countryside.
Well, I’m 35 and have used ADFs my entire flying career. Granted, my first two years of operational flying for Uncle Sam were in Central and South America where ADFs are much more common. After that it was mostly for training, with the occassional no-kidding approach in Africa to keep you on your toes.
I finished up teaching at a pilot training base, and we taught students to fly ADF/NDB approaches. These are the students who are going to heavies, and we taught them in the T-1. It was interesting to fly NDB approaches using a glass cockpit - it made things a lot easier when your ADF needle is taking up the middle of an 8-inch screen!
I think I have a decent amount of ADF experience for an Air Force pilot, but that’s just because of where I flew. Now I only use ADFs as marker beacons. In three and a half years of airline flying I’ve seen ONE ADF approach, and that was in the Dominican Republic.
Sam Stone it sounds like that PAR you got turned into a “no-gyro” approach! A normal PAR has the controller giving you headings to fly. If you lose your navigation ability for some reason they can give you no-gyro vectors, which sound like what you had: “Turn left…stop turn”, etc. Also, the PAR controller usually coordinates your landing with the tower and issues landing clearance to you. That way you’re not mucking around in the soup trying to change a radio frequency when he says “over landing threshold”.
I have to call you on this - or are you just exaggerating?
The contrail should stream out behind a jet aligned with it’s longitudinal axis (i.e. straight behind), regardless of the wind or crab angle. The jet and contrail are flying along in the same moving body of air, after all. Of course, the direction of the flight path may be different, as you’ve noticed, but the angle of the contrail to the flight path should be the same as the crab angle.
The only way for the contrail to string out anywhere near perpendicular to the flight path would be if the wind speed was nearly equal (but less than) the airspeed of the aircraft.
If the airplane’s ground speed were the same as the wind speed, (making the plane’s airspeed sqrt(2) or about 1.4 times the wind speed) then the contrail would string out at an angle of 45 degrees to the flight path. I’d say that’s pretty extreme.
Or, maybe you saw something different than this idealization. Maybe the jet was climbing thru a wind shear?
Pilot141: Heh. I told you I was dredging up a 15 year old memory. But you’re right - I mutated that into a ‘no gyro’ approach. You’re also right about the PAR controller handling the landing clearance and such.
Some lazy pilots used to ask for ‘practice’ PARs when it was hazy or the sun was in their eyes and they didn’t want to go looking for the airport. Or sometimes they’d ask for a ‘practice’ PAR when they were lost. One day we were sitting in the tower (we had our own ‘tower’ at CFB Edmonton - it was the ground controller’s tower back when our ramp was used by the U.S. for servicing B-52’s), when we heard someone call in on the radio, “Tango Kilo Kilo requesting a practice radar approach.” The controller responded, “Negative Tango Kilo Kilo, we’re too busy for a practice approach right now.” Pause for about 10 seconds, then, “Namao, Tango Kilo Kilo would like to request a REAL radar approach…” Much laughter in the building.
