Welcome, Guest. Please login or register.
Did you miss your activation email?

News: The latest PSX update (version 10.54 from 7 November 2018) is available at: http://aerowinx.com/board/index.php?topic=4191.0

Author Topic: Tropical cyclone  (Read 416 times)

Hardy Heinlin

  • Moderator
  • Join date: May 2009
  • Posts: 9703
    • Aerowinx
    • Email
Tropical cyclone
« on: Thu, 29 Nov 2018 15:19 »
Good evening,

a question for the jet pilots: Are there any significant performance or instrument effects when overflying a tropical cyclone ? (Excluding the effects of the CBs which you certainly avoid.)

If the pressure in the cyclone center is, say, 967 hPa, and 100 nm further it's 1000 hPa, then the difference is 33 hPa, i.e. 1000 ft. Say, groundspeed is 600 kt. 100 nm are passed in 10 minutes. Climb or descent 1000 ft on the true altitude scale in 10 minutes, i.e. 100 fpm. You are near MAX ALT at FL450.

Assume the whole overflight is flown at FL450. The air pressure and density will remain unchanged. But the true altitude will change: It will decrease when approaching the center and will increase when leaving it.

The air pressure and density will not change and therefore will not change the aircraft performance in this regard; The decreasing lift will automatically make the aircraft descend on the true altitude scale, and vice versa when the lift increases.

But what about the geometric vectors in such an extreme case? The 744 V/S indicator uses a mix of barometric data and IRS data. The IRS data is used for short term variations. The barometric data is for long term stability. Would the V/S indicate 100 fpm in the above example while the altitude tape is fixed at 45000?

I'm also wondering whether there may be subtle effects on the aircraft performance even though the air mass per se won't change. The variable lift will draw the profile automatically; but there are also geometric, gravitational potentials when descending and climbing geometrically within the same pressure altitude. It might be a factor when flying at FL450 where 100 fpm means a lot ...


Regards,

|-|ardy

jcomm

  • Join date: Dec 2011
  • Location: Portugal
  • Posts: 1089
Re: Tropical cyclone
« Reply #1 on: Fri, 30 Nov 2018 18:18 »
Hardy,

you're assuming the same horizontal gradient at FL450 that you would experience at lower / ground level...

It would probably be almost imperceptible at those higher levels. Also the assumption regarding density is not correct.

Would rather suggest a look at the new "Space Weather" ( basicaly due to Solar activity ) operational directives as described in the latest Anex 3 release ( dated July 2018 ), namely under section 3.8, with interesting and worrying possible effects not only on various systems ( HF, GNSS, SatCom ) but also crew and pax exposure to the radiation.

On the other subject of radioactive cloud, the SIGMET specifically forbid traversing such zones, so, it would be very unlikely to see a 744 traversing such an area, unless the thing had just happened ...
« Last edit: Fri, 30 Nov 2018 18:37 by jcomm »

Hardy Heinlin

  • Moderator
  • Join date: May 2009
  • Posts: 9703
    • Aerowinx
    • Email
Re: Tropical cyclone
« Reply #2 on: Fri, 30 Nov 2018 18:33 »
My assumption regarding density was just fictitious for my above ideal example which didn't include temperature variations.

Will

  • Join date: May 2009
  • Location: Chicago
  • Posts: 1658
    • Email
Re: Tropical cyclone
« Reply #3 on: Fri, 30 Nov 2018 19:38 »
We might be able to test this, if we could get those static ports plugged. ;-)
Will /Chicago /USA

Simonijs

  • Join date: Oct 2014
  • Location: Amsterdam
  • Posts: 72
Re: Tropical cyclone
« Reply #4 on: Sat, 1 Dec 2018 17:38 »
So, for me to get this right (because I may be very wrong)..:

The aircraft is surfing along the 148,165 hPa pressure level, relative to 1013,25 hPa. This translates to a pressure altitude of 45.000 ft on the altimeter; the indication won't change as long as the outside pressure measures 148,165 hPa. With no change in outside static pressure, there will be no barometric vertical speed.

I do not yet understand this conclusion: “the decreasing lift will automatically make the aircraft descend on the true altitude scale”. Why would lift decrease if performance, air pressure and density remain the same?

For the true altitude [TA] scale we could have a look at the ICAO – International Standard Atmosphere [ISA] to read and use values in terms of geometrical altitude for p, ρ, T and g. Only T is a constant value (216,65K), the others vary with geometrical altitude. 
 
In order to be able to fly at MAX ALT, the aircraft mass will be around 200.000 kgs. Let’s move from “high to low”. At the point where SLP = 1000 hPa, TA will be ± 44.700 ft. The ISA value for g at 44.700 ft geometrical altitude = 9,6702 m/s2 - resulting in an aircraft weight of 1934040 N.
At the point, where SLP = 967 hPa, the TA will be ± 43700 ft; the value for g at 43.700 ft geometrical altitude = 9,6732 m/s2, resulting in an aircraft weight of 1934640 N. So: slightly more W at the lower geometrical altitude. If Lift is momentarily set to a fixed value, my guess would be: on the TA scale, the aircraft descends slowly because of L, becoming slightly less than W. And maybe the IRS will pick this up, but I doubt it since Δ g is very small (1,0003 over ten minutes = long term). My bet: there will be no indication of Vertical Speed.

Yet another effect will be that this 200T aircraft is consuming fuel; in ten minutes ± 1130 kgs (or 10960 N – Freighter), or ± 1160 kgs (11217 N – PAX/Combi). As Jcomm mentioned, the real atmosphere will deviate from the one formulated in ISA: already on a daily basis, and significantly near a tropical cyclone. Confusing many variables. But…: is has been a long time, since I was at school.

Simon

Hardy Heinlin

  • Moderator
  • Join date: May 2009
  • Posts: 9703
    • Aerowinx
    • Email
Re: Tropical cyclone
« Reply #5 on: Sat, 1 Dec 2018 18:34 »
I do not yet understand this conclusion: “the decreasing lift will automatically make the aircraft descend on the true altitude scale”. Why would lift decrease if performance, air pressure and density remain the same?

I was trying to describe a level flight maintaining a true altitude above MSL (geometric, not barometric). Imagine you're flying from Amsterdam to London at 45000 ft true altitude. There is an air column over Amsterdam, and there is a vacuum column over London. When you reach the vacuum over London at 45000 ft geometric altitude, the lift will decrease to zero, and therefore you will descend to lower geometric, true altitudes.


|-|ardy

Will

  • Join date: May 2009
  • Location: Chicago
  • Posts: 1658
    • Email
Re: Tropical cyclone
« Reply #6 on: Sun, 2 Dec 2018 15:18 »
I'm not confused by the idea of flying along at a constant pressure altitude while the true altitude goes up and down, but I am intrigued by the idea of the vertical speed indicator getting IRS input. Do we know the logic here?


What would happen if we taped up the static ports, teleported the aircraft to FL450 with zero airspeed, and let it free fall down? What would the VS indicate?
Will /Chicago /USA

Hardy Heinlin

  • Moderator
  • Join date: May 2009
  • Posts: 9703
    • Aerowinx
    • Email
Re: Tropical cyclone
« Reply #7 on: Sun, 2 Dec 2018 16:06 »
The VS would indicate IRS vertical motion, i.e. the speed vector towards the center of the earth. The vertical barometric rate will dampen the VS indication in the long run if the barometric rate differs from the inertial rate.

I think the ratio of baro/inertial is similar to that of the attitude indicator's gravity/inertial ratio. The pendulum weight is for long term attitude stabilization using the gravity vector towards the earth's center, and the gyro indicates short term attitude changes.


|-|ardy

torrence

  • Join date: May 2009
  • Location: Altadena
  • Posts: 403
    • Email
Re: Tropical cyclone
« Reply #8 on: Thu, 6 Dec 2018 02:25 »
What would happen if we taped up the static ports, teleported the aircraft to FL450 with zero airspeed, and let it free fall down? What would the VS indicate?

What I love about this forum.  Not sure Boeing would ever come up with such a thought experiment (Gedanken auf Deutsch). Maybe they should if they're building hard-to-disable angle-of-attack 'protections' into current systems re recent 737MAX problems.

Cheers
Torrence
Cheers
Torrence