Wake Turbulence: Small behind a Large

D

DCR

Pre-Flight
Joined
Jun 14, 2023
Messages
44
Display Name
Display name:
rudy
Hello,

Just reading AIM section 7-4-9 and noticed there is no guidance for a small taking off behind a large, if both are making a normal departure from the threshold.

This is a common situation I encounter at my home base, and usually I just hit the gas when ATC releases me. However, it is a Delta so separation is completely on me (pretty sure about this - correct me if wrong).

When driving your trusty ol' C172, what do you think is a safe time to wait if departing behind an A320/321, B737, Gulfstream V, Bombardier (CL60), etc.?? Would / do you ask for a delay from ATC?

PS: There is guidance for opposite direction or intersection takeoffs, but I have never seen that.

PPS: Those are all "Large" aircraft per FAA's definition which I see at my base.
 
Wake vortices sink and drift downwind. I recall being taught that, under most loading conditions, our aircraft types will lift off sooner and climb at a steeper gradient (ft/NM, not ft/min) than almost anything that could make a wake strong enough to concern us. As a matter of extra caution, you could turn 10 degrees upwind until you're far enough away to no longer be concerned.
 
Nah, unless you’re behind a 757, separation is basically on you from a threshold take off. ATC is using 6,000 ft SRS sep for you (cat I) behind the large (cat III) but that’s about it.

Threshold and opposite direction can be 2-3 minutes depending on type of large but some of that’s waiverable.

If you want more time, just say you’d like 2 minutes for wake turb. Unless the airport is super busy, they shouldn’t fuss.
 
Last edited:
Good points.

Do you make any special adjustments if following a large on approach to land? The recommended separation there is 4 nm, but I don't even use an EFB (yet - working on it) so I'm just eyeballing everything.
 
DJTorrente said:
Wake vortices sink and drift downwind. I recall being taught that, under most loading conditions, our aircraft types will lift off sooner and climb at a steeper gradient (ft/NM, not ft/min) than almost anything that could make a wake strong enough to concern us. As a matter of extra caution, you could turn 10 degrees upwind until you're far enough away to no longer be concerned.
Click to expand...
Hence, takeoff or approach, stay above a big, heavy's flight path.
 
DCR said:
Good points.

Do you make any special adjustments if following a large on approach to land? The recommended separation there is 4 nm, but I don't even use an EFB (yet - working on it) so I'm just eyeballing everything.
Click to expand...
I stay above their glide path and touch down after their touch down point.
 
DCR said:
Hello,

Just reading AIM section 7-4-9 and noticed there is no guidance for a small taking off behind a large, if both are making a normal departure from the threshold.

This is a common situation I encounter at my home base, and usually I just hit the gas when ATC releases me. However, it is a Delta so separation is completely on me (pretty sure about this - correct me if wrong).

When driving your trusty ol' C172, what do you think is a safe time to wait if departing behind an A320/321, B737, Gulfstream V, Bombardier (CL60), etc.?? Would / do you ask for a delay from ATC?

PS: There is guidance for opposite direction or intersection takeoffs, but I have never seen that.

PPS: Those are all "Large" aircraft per FAA's definition which I see at my base.
Click to expand...
AC 90-23G
 
  • Like
Reactions: DCR
Get off the ground before the big one left the ground. Offset slightly up wind.

If any doubt, ask for a delay in takeoff. Nothing wrong with that. WT can destroy your airplane and kill you and your passengers. Make decisions that keep you safe, and free from a lifetime of guilt.

At a local airport I have had the controller hold me for a few minutes after a heavy took off.
 
I always ask for a delay if taking off or landing behind a big, heavy airplane. I once had a controller being an a** about it but other than that one time, they’ve always been super nice and accommodating to my request.
 
I don't trust the accountability of any others besides myself. I was taught wake vortices sink at 500 ft/min and dissipate in about 2 minutes. I'll ask for a delay or immediate turn if I'm right behind something big - since I probably can't out climb its departure. I had to reject take off clearance once because tower couldn't give me an immediate turn behind a Global 7500 and wanted me to go while it was still over the departure end of the runway.
 
I've hit wake turbulence at altitude. Small 737 making approach to KPAE (Paine Field, WA) six or seven miles from the runway. I flew under its flight path at right angles about 60 seconds later heading west to Whidbey Island. It was a sudden roll excursion, compensated for. It was dangerous only in that it was surprising.

It could easily have been much more serious if I had crossed the 737's flightpath sooner or had been close to the ground.
Wake turbulence from even a small heavy can toss one of our little spamcans around violently. I'd rather **** off ATC than take the risk.
 
Note the previous aircrafts rotation point and make sure to rotate before then. After that your climb angle in a GA plane should be steeper than any commerical level aircraft.
 
You should be able to get airborne within 100 ft and climb at 10,000 fpm but most of the time that won’t happen.
 
MauleSkinner said:
Why should it be? Is that like saying you should be able to outrun the cop’s radio, but most of the time you won’t?
Click to expand...
You shouldn't be able to outrun the radio unless you're special, and that's why most of the time you won't.

The FAA says your climb path should be before and above the preceding aircraft's climb path, but in a single-engine GA plane taking off after a jet or a multi most of the time you won't be able to do that.

That's what I meant.
 
Revtach said:
You shouldn't be able to outrun the radio unless you're special, and that's why most of the time you won't.

The FAA says your climb path should be before and above the preceding aircraft's climb path, but in a single-engine GA plane taking off after a jet or a multi most of the time you won't be able to do that.

That's what I meant.
Click to expand...
Fortunately the FAA says nothing about a GA airplane that should be able to outclimb a jet or multi engine airplane.
 
I don't seem to see much actual experience about hitting wake turbulence on take off. I think despite what it looks like, that as has previously been mentioned here most small aircraft actually climb at a better ft per nm than the larger aircraft. Most of us small guys are off the ground in 500 ft and climbing while none of the big guys are any where close to that so you have a good start. Is also possible that it isn't quite as bad on departure because the big aircraft is not as dirty (less flaps and drag devices) than when they are landing.

Also perhaps a bit counter intuitive, but is probably better climb at Vy or bit higher than at Vx if thinking you might hit wake turbulence, sort of like the recommendation when landing to carry some extra speed in turbulent conditions, in part because your controls, especially the ailerons are quite a bit more effective with a bit more speed.

As for landing if you look for the you tube examples you will see that almost all of them are slow approaches with full flaps, sort of the worst case situation to hit wake turbulence where you have very little climb performance when you add power until you retract the flaps that the ailerons are the least effective at the slower speed. Same if you think wake turbulence is a possibility, treat like windy turbulent conditions. Increase your approach speed and use less flaps along with the AIM recommendation

Brian
CFIIG/ASEL
 
brcase said:
I don't seem to see much actual experience about hitting wake turbulence on take off. I think despite what it looks like, that as has previously been mentioned here most small aircraft actually climb at a better ft per nm than the larger aircraft. Most of us small guys are off the ground in 500 ft and climbing while none of the big guys are any where close to that so you have a good start. Is also possible that it isn't quite as bad on departure because the big aircraft is not as dirty (less flaps and drag devices) than when they are landing.
Click to expand...
I’ve never gotten a takeoff in where wake turbulence was a potential issue that the tower didn’t give me a fairly quick turnout to avoid wake turbulence. Despite the head start, I’ve never flown a light GA plane that will outclimb a jet.
 
"Out climb" can mean multiple things. "Angle of climb" or "Rate of climb." In this case, the applicable specification is the Angle of climb between your average GA spamcan and a big heavy. Also a GA plane lifts off thousands of feet before a heavy. That also factors into the angles calculation.
 
Modern airliners have super efficient wings, and throw out some wicked wake turbulence. Initial climb rates of 4000fpm or so will place them ahead and above the small GA airplanes in a hurry, especially with winds down the runway.

So it boils down to the entrail small GA airplane "Do you feel lucky?"
 
MauleSkinner said:
I’ve never gotten a takeoff in where wake turbulence was a potential issue that the tower didn’t give me a fairly quick turnout to avoid wake turbulence. Despite the head start, I’ve never flown a light GA plane that will outclimb a jet.
Click to expand...
long term I agree, but for that 1st 2 to 3 miles it think it would be interesting to see some actual data comparisons.
For a fully loaded 172 M at 5000ft 41 degrees no wind it looks like will be at 50 ft in 2480ft. so for 10,000 runway typical for bigger jets I still have 7500 ft to climb before getting to the end of the runway. Rate of climb looks like 435ft/min at 85mph. 7500ft = 1.42 miles. At 85mph 1.42 miles should take 1 minute. putting me crossing the far end of the runway at about 435 (maybe 485) feet. OK a big guy is probably equal or beating me by a bit, but I am just guessing on the a 737 performance.

a more typical 2 people (2000lbs) at 2500ft and 50 degrees F. I am clearing the 50ft at 1325ft. and climbing at ~710ft/min. 8675ft remaining to climb = 1.64 miles. 1.64 at 85mph = 1.16minutes, putting me at 823 ft at the end of the 10,000 runway. I would large aircraft aren't that high at the end of a 10,000 foot runway. Of course a lightly loaded one would easily do it, but they don't want to fly them empty and at lighter weight the wake turbulence would be less.

Just me thinking out loud and playing with some numbers. Maybe someone has access to 737 or similar numbers.

Brian
 
brcase said:
long term I agree, but for that 1st 2 to 3 miles it think it would be interesting to see some actual data comparisons.
For a fully loaded 172 M at 5000ft 41 degrees no wind it looks like will be at 50 ft in 2480ft. so for 10,000 runway typical for bigger jets I still have 7500 ft to climb before getting to the end of the runway. Rate of climb looks like 435ft/min at 85mph. 7500ft = 1.42 miles. At 85mph 1.42 miles should take 1 minute. putting me crossing the far end of the runway at about 435 (maybe 485) feet. OK a big guy is probably equal or beating me by a bit, but I am just guessing on the a 737 performance.

a more typical 2 people (2000lbs) at 2500ft and 50 degrees F. I am clearing the 50ft at 1325ft. and climbing at ~710ft/min. 8675ft remaining to climb = 1.64 miles. 1.64 at 85mph = 1.16minutes, putting me at 823 ft at the end of the 10,000 runway. I would large aircraft aren't that high at the end of a 10,000 foot runway. Of course a lightly loaded one would easily do it, but they don't want to fly them empty and at lighter weight the wake turbulence would be less.

Just me thinking out loud and playing with some numbers. Maybe someone has access to 737 or similar numbers.

Brian
Click to expand...
I doubt that all-engine book numbers exist for 737 takeoff since Part 25 takeoff performance calculations are predicated on an engine failure, but they do publish a balked landing climb gradient that’s all-engine. For the Beechjet at max takeoff weight, sea level, 20 degrees C, that works out to about 14%, and that assumes full flaps and gear down, which probably means well above 20% after gear retraction on takeoff. 435ft/min at 85mph Works out to about 6%. So we’re talking about less than 3 miles from the end of the runway where you cross my wake, even with the difference in takeoff distance. And I’m pretty sure 737s climb steeper than the Beechjet.
 
I was taught to ensure takeoff before the big jet rotation point (should be a non-factor in a light single) and sidle or turn upwind after takeoff if possible or with ATC permission.
 
Doc Holliday said:
Modern airliners have super efficient wings, and throw out some wicked wake turbulence. Initial climb rates of 4000fpm or so will place them ahead and above the small GA airplanes in a hurry, especially with winds down the runway.

So it boils down to the entrail small GA airplane "Do you feel lucky?"
Click to expand...

I thought this is the old "rate of climb" vs "angle of climb" discussion.

No sane person would claim a GA aircraft can climb faster, but I thought they do climb steeper than the large and heavy guys. Too tired right now but I think some simple math will confirm this using published rate of climb and departure speeds if I can find them to calculate respective angles. Will be an interesting exercise at least.

Of course, if you level off and they are still climbing then yes, you would fly right through their wake regardless of the angle of climb result. Given the altitude they climb to this seems a very probable situation.
 
DCR said:
I thought this is the old "rate of climb" vs "angle of climb" discussion.

No sane person would claim a GA aircraft can climb faster, but I thought they do climb steeper than the large and heavy guys. Too tired right now but I think some simple math will confirm this using published rate of climb and departure speeds if I can find them to calculate respective angles.
Click to expand...
Simple math has already confirmed the opposite.
 
MauleSkinner said:
I doubt that all-engine book numbers exist for 737 takeoff since Part 25 takeoff performance calculations are predicated on an engine failure, but they do publish a balked landing climb gradient that’s all-engine. For the Beechjet at max takeoff weight, sea level, 20 degrees C, that works out to about 14%, and that assumes full flaps and gear down, which probably means well above 20% after gear retraction on takeoff. 435ft/min at 85mph Works out to about 6%. So we’re talking about less than 3 miles from the end of the runway where you cross my wake, even with the difference in takeoff distance. And I’m pretty sure 737s climb steeper than the Beechjet.
Click to expand...
Yes. And while I'm one to get all mathy and spew data everywhere, the simple fact is that I've never been in a turbine that didn't climb out the top of the inverted wedding cake. If you're going 180 knots and getting 3,000 fpm, or 240 knots and 4,000 fpm, that's 1000 feet per nautical mile. However, in my Mooney, on a good day in the winter by myself I'll be getting 1500 fpm at 120 knots, which is only 750 ft/nm. So even if I take off a mile before the big bird, I'll cross their vertical path in 3 minutes/6 miles.
 
On a dead calm day ...
Since wake turbulence vortices spin off the wingtips, spread out, and sink, what sort of turbulence is directly under the heavy's fuselage?
 
Domenick said:
On a dead calm day ...
Since wake turbulence vortices spin off the wingtips, spread out, and sink, what sort of turbulence is directly under the heavy's fuselage?
Click to expand...

Ha! Are you suggesting to shoot the gap? :)
 
You must log in or register to reply here.
Back
Top