View Full Version : Exhaust velocities

8th Mar 2004, 07:48
First post, Please excuse my ignorance, if I'm asking something stupid...

I am interested to know any info/website where i can get data on exhaust velocities behind B737/500 at takeoff. I would like velocities up to 300m away

What is considered a safe speed for a public access area? eg, a public road x metres (say) from end of runway experiences 40mph (say) 'jet blast' - is that ok or too much?


8th Mar 2004, 09:42

Tell us a little about the story relating to Joe public getting too close to the jet blast you are going to report on....:}

8th Mar 2004, 11:03
I have a book which suggests at 30m behind a jet aircraft, the thrust stream will be 25kts at idle and 125kts at full power. I don't know how accurate that is and it doesn't mention a specific aircraft/engine.

"Jet blast is powerful" (http://www.dellanave.com/files/Fod5.mpeg)

8th Mar 2004, 11:18
Try it and see :E

There is a good video of an Air France 747 taking off from
St maarten where people on the beach hundreds of feet
away are blown into the water...sorry no link to the video

8th Mar 2004, 12:53
Rent the movie "Pushing Tin"...
Knowing suspension of disbelief only goes so far...


8th Mar 2004, 14:54
My old training notes for the B737Classic fitted with CFM56b2 powerplants shows the exhaust hazard area extending 100ft(30 meters) from the TAIL of the aircraft at IDLE thrust.At BREAKAWAY power this extends to 510ft(155meters).At takeoff power it is 1900ft(579meters).Sorry i don`t have any exact exhaust veloicities.
Regards DDG

8th Mar 2004, 17:35
Paracab....Ok a bit more info.. Theres one sleepy little midlands airfield we go to, where theres a public road directly at the end of the runway, about 200 m back. Not sure what you experts would say about hanging around in such areas, but it is a public area and you get a great view and good photos. Now my point is they are going to start these 737's operations shortly. I would have thought 200m in the line of fire was pretty close for a public area?

DDG - sorry for being ignorant but whats 'breakaway' power? At the end of the runway at start of takeoff does the 'takeoff' power apply?

8th Mar 2004, 18:41
Can't help with the speeds for the moment (I'm in a hurry) but the mass of air exausted from a CFM56-3-B1 is 297 Kilograms per second (note: just one engine)... :oh:

9th Mar 2004, 06:45
Breakaway power is the amount of power used to start to get an aircraft to taxi,(ie breakaway from standstill).
This is a Boeing term used within their manuals.
As each environment/handling pilot will be different this figure i imagine would be an average used as a guideline.
The take off power hazard area i listed is for static take off power as used for ground running purposes,so in your example it would apply from the end of the runway(panio keys) .
If you have access to the aircraft mantenance manual the figures and warnings are listed in mm71-00-00-201 Engine ground safety precautions.Note the AMM doesn`t list exhaust velocities within the hazard area.
Regards DDG

9th Mar 2004, 14:39

Go to http://www.geocities.com/profemery/aviation.html

Down the page - in the ACN chart section - is a chart of the 737-800 jet blast at TAKEOFF THRUST, not breakaway thrust.

Typical rules are:

maximum wind velocities which people, objects and buildings in the vicinity of an aeroplane may be subjected to must not be more than:
(a) passengers and main public areas, where passengers have to walk and people are expected to congregate . 60kph;
(b) minor public areas, where people are not expected to congregate . 80 kph;
(c) public roads . 50kph where the vehicular speed may be 80 kph or more, and . 60kph where the vehicular speed is expected to be below 80 kph.
(d) personnel working near an aeroplane . 80kph;
(e) apron equipment . generally not in excess of 80kph;

I got to tell you even 50 kph feels like a lot more. Saw some tests recently using a hand-held anemometer from Bureau of Meteorology. Here is what was written: "Pilot was requested to use maximum power takeoff consistent with safety for the purpose of measurement. 737-300 aircraft. Maximum measured jet blast was 45 kph with instantaneous gust to 50 kph. Note that there was significant dust/sand blast, and without the benefit of the anemometer it would have felt to be a higher blast velocity than actually measured."

In normal speak it really feels like you're being blasted even at these modest blast speeds. This test wasn't unsafe at all just [forgive me for this] a real blast.

Boeing jet blast data for the 737-300 (and 400 series) (D6-58325-2; July 1990) shows 56 kph blast contour to go out to 155m for breakaway thrust. The 56 kph takeoff power contour goes to 579m (geez - same as what DDG was quoting - nice to find two technical documents agreeing). Back to the tests "The breakaway thrust contour is consistent with the measured maximum blast velocity, and supports the use of breakaway thrust for blast design".

So 200m would seem to be pretty reasonable. There are airports which have got public areas which are closer. Some of them stick up blast walls, some don't. Some do, and the photographers got annoyed because they can't see the aircraft. Some don't and no-one complains about the blast.

Just looked at the movie that ROB-x38 linked to. Great clip.

The van looks to be about 20m behind the tail - maybe a little less. From the 737-800 blast graph, and assuming that the United had engines wound up to takeoff thrust for the demonstration, that works out to be 300-325 km/hr jet blast. Up close like that, jet bast is a killer.

That blast speed is faster than then the winds in a bad cyclone. Explains why the van got trashed. Wonder if the van was a rental.

9th Mar 2004, 17:46
Thank you very much for your detailed replies. Thats just what I was after.:D

Looks like I might need some goggles, and stand to one side in the future!:cool:

Mark 1
10th Mar 2004, 00:28
At the nozzle exit, the velocities will be around 1400-1500 ft/sec at the hot nozzle and around 1000 ft/sec at take-off power (just below locally sonic conditions).
For the long cowl mixer nozzle type engines the hot jet rapidly diffuses with the bypass flow.
Further downstream the exhaust will diverge at about 7 degrees.
I don't know if this would hold up for 300m, but if it did the jet would be over 30m in diameter, so about 400 times the area at source and so 1/400th the velocity.
I recall a test some time ago for designing an engine test bed where a pitot rig was used to measure the blast at about 50m behind a jet engine at full power. All of a sudden all the sensors read zero due to the entire rig complete with several blocks of concrete being blown into the next field.
Hope this helps.

14th Mar 2004, 07:45
Mark 1
>Further downstream the exhaust will diverge at about 7 degrees.
I don't know if this would hold up for 300m, but if it did the jet would be over 30m in diameter, so about 400 times the area at source and so 1/400th the velocity.

If, hypothetically the exhaust did *NOT* diverge, what would the calculation be to determine the distance where the velocity drops to zero ? Just approximately would be interesting enough for me.

I'm guessing that the air density / viscosity would be part of the calculation and probably the friction over the ground. What if we ignore the friction over the ground ? .. is it just a question of the velociity, viscosity, density all part of some kind of formula ?


Mark 1
15th Mar 2004, 14:19
If the jet did not diverge then it would go on forever.
The equation for mass flow in a stream tube is:
Density * Area * Velocity = Constant

So not a good assumption for calculations.

My guess would be 100-200 nozzle diameters to dissipate the jet from a critical nozzle.

Ranger One
15th Mar 2004, 15:32




16th Mar 2004, 07:24
Ahhh thats the bugg*r, cheers Ranger

If that doesnt put you off nothing will!:D