Downdrafts - built-up confined areas

[Little Bird]

I'm looking for comments/experiences regarding downdrafts in built-up confined areas.

I would be arriving light, but departing heavier (close to MAUW).

The LZ has a high-rise to the south-west, low-rise to the north, and then trees or lower buildings (not less than 20 feet high) west and east, south-east - winds are predominantly from the s.w./west.

Cheers

[Wally]

What kind of winds you talking about?

How close to the helipad are the structures?

It's a surface pad or roof-top?

How long will you be on the pad?

How much power margin HOGE?

 

Prevailing winds can deceive, local flows can vary direction and speed. 5-10 knots might generate some downwind turbulence, and if you're close in to a significant obstacle it can surprise you. Higher winds can generate a "shadow" of relatively calm air in the lee, but bigger issues where that meets the flow.

A rooftop, standing in the turbulence and generating ups and downs around it can also be interesting. A surface helipad will almost always be easier if there's not significant vertical in and out.

If you're on and off and new to the pad situation, a low slow pass can let you get a feel for where the issues are. If you have sufficient power for a vertical ascent to clear air and good forced landing areas, generally not an issue.

[Gomer Pylot]

There is no way to tell you anything definitive. If there are downdrafts, then there will also be updrafts. Can't have one without the other. Downdrafts tend to happen around hotter surfaces, such as parking lots, but it's really hit or miss. If there is a downdraft over one place, there will be an updraft somewhere nearby. About all I can tell you is to make your approach slow, and hope for a clear departure path. Try to depart into the wind, or as much into the wind as possible, and be smooth on the controls. Torque spikes are generally the result of control roughness, and that needs to be avoided. You also may get swirling wind as it goes around obstacles, so be alert for that. Without knowing a LOT more, about all I can advise is to be slow and smooth, and you should be able to get out. But do try to calculate the density altitude, and check the performance charts to see if the power margin should be available.

[Goldy]

There is no way to tell you anything definitive.

That's where I was headed......take it in nice and slow and observe the effects of your surroundings. Then taking off, you might be able to use that knowledge to get you in the air quicker. Every location is so different and of course, all dependent on wind direction.

[Little Bird]

Thanks for your thoughts and replies thus far.

The site is a surface LZ, cut grass, approx. 75m x 50m in area.

The low-rise is right next to the northern boundary, the high-rise is 130m to the s.w. Trees and other lower buildings are 30m west, east and s.e.

Winds at this time of year are 12-18kts 220 through 270 degrees.

DA is just over 2000ft. Operating a AS350B2 model. Operations are to land empty and depart 10-15 minutes later (after shut-down) with 4-5 pax and 60-80% fuel.

Is there a AS350 rule of thumb for power checks?

Cheers

[helipilotm]

Don't take this the wrong way but that isn't a confined area. 2000 ft density altitude is nothing. I'm assuming your not the one flying correct? There shouldn't be much of a problem to get in and out of there unless its littered with wires. If there is a problem the pilot probably shouldn't be flying.

[Wally]

Thanks for your thoughts and replies thus far.

The site is a surface LZ, cut grass, approx. 75m x 50m in area.

The low-rise is right next to the northern boundary, the high-rise is 130m to the s.w. Trees and other lower buildings are 30m west, east and s.e.

Winds at this time of year are 12-18kts 220 through 270 degrees.

DA is just over 2000ft. Operating a AS350B2 model. Operations are to land empty and depart 10-15 minutes later (after shut-down) with 4-5 pax and 60-80% fuel.

Is there a AS350 rule of thumb for power checks?

Cheers

 

The performance charts for the B2 are a very good place to start, reliable indicators if it's a healthy engine. You will NG limit before TQ warmer than ISA.

Cockpit rule of thumb: 1% NG and/or 3% TQ is 200 lbs. Do that HOGE and remember passengers all remember their weight in middle school and don't have a clue how much junk they're carrying.

 

4-5 pax and 60-80% fuel is a pretty wide range in the area of 400 lbs. There's 36-40 minutes variation in fuel proposed...?

Specifics given look easy if you're normally competent and the aircraft as outfitted isn't too heavy, keep it under max MGTO.

Edited August 25, 2013 by Wally

[Little Bird]

Thanks Wally