Friday, February 01, 2008

Crossing 600 miles of water to get to Bahamas.

I like that pic. I don't know how it's going to look on the blog, but at home it looks like I can actually look up and see into space.

These shots were taken a short while ago in a Citation Bravo, which is the newfangled version of the Citation II I fly - the airframe is the same, but the engines have 10% more thrust, the landing gear is trailing link (makes for softer landings) and the cockpit instruments are a lot more modern.

We took this plane to Bahamas. To be honest, I am less excited about that area than I used to be - there are only so many days I can walk along the beach with a pina colada before I start to pine for my couch at home. It's weird, but true.

Anyway, where were we?

Right, Bahamas.

I'm posting these pics as I'm looking outside my window at a massive snowstorm which is hammering the Toronto area. You know, the part where I previously said I was sick of Bahamas? I take it back. I'm ready for another trip, Mr. Charter client sir.

Okay, I'm getting off track. I actually had something interesting to post today, so now that I have alienated 90% of you, it's time to talk about pilot-geek stuff and chase away the remaining 10% :D

On the way back, the headwinds were such that in order to have enough fuel for the return leg, we had to climb to 43,000' above sea level, where the fuel burns are lowest. I'm not generally a huge fan of high-altitude flight (my personal feel-good altitude is 33,000'), mostly because we get exposed to a lot of solar radiation, which isn't particularly good for us. But there's another reason too, one which I will attempt to explain.

The higher we go, the more we tuck ourselves into 'coffin corner'

Here's a picture illustration of what I mean, taken at 38,000', climbing to 39,000'.

The part we care about at the moment is the airspeed indicator, which is at the top left of the pic. We are indicating 184 knots, and climbing at 500 feet per minute. Now see the little red bar at the top of the airspeed indicator, starting somewhere at around 218 or 219 knots? That's our never-exceed-speed indicator, showing the fastest we are allowed to go before we start to lose some structural strength in the airframe. To oversimplify - if we go too far over our redline, the wings come off.

Now there is another bar on the airspeed indicator that we can't see yet - it's a white one which appears at the low end of the indicator. It shows us the stall speed of the aircraft at that particular altitude. At 38,000', it's not yet an issue.

Now let's climb up a little higher.

See what happened to that little red bar on the airspeed indicator? We only climbed 3,000', but now the red bar is showing that our never-exceed speed is only about 202 knots. You still can't see it, but the white bar showing our stall speed has climbed also.

Let's push our luck and climb to 42,000' now. The altitude that we are currently flying at is indicated on the right side of the display, in green. The altitude at the top of the right side in red is what we have told the plane to climb to.

Now the red never-exceed-speed bar is showing somewhere around 198 knots, and we see the first appearance of the white dont-go-below-this-or-you-will-stall bar, which is showing around 137 knots or so.

I found it interesting that in these pics, we have only climbed 3,000'

We still have 60 knots to play with in between these two ranges, so it's not a huge issue for us, but consider this: At sea level, our never-exceed speed would be around 275 knots, and our stall speed would be around 85 knots, which gives us a 190 knot speed range that we can safely operate the airplane in.

This same effect applies to all aircraft - it was said that the US military spy planes such as the U2 or the SR71, who climb to altitudes above 60,000', would only have a 5 - 15 knot window in between the never-exceed speed and the stall speed, so it was vital to handle the aircraft extremely gently to avoid an upset.

Why does this happen? Well, I'm not going to go into it in much depth - Aviatrix at Cockpit Conversation can explain it a lot better than I can, but the short version is that the air is less dense at altitude, which causes all sorts of funky things to happen to airplanes.

That's it for my pilot-geek stuff today, I just thought it was interesting to see the effect of lower air density in real life.


Smurfjet said...

I was wondering when you were going to take back the 'not enjoying the sandy beach with a pina colada' stuff.

I was just reading this article about the upcoming storm, it has a very striking picture...

John said...

How did you like the Bravo compared to the II?

Anonymous said...

Why would structural-strength-speed decrease as the air gets thinner? Could it be that the upper boundry is really the Mach limit? That is a number that decreases with altitude.

Sulako said...

You are correct - I didn't want to get into a detailed explanation at all, and I left it at "the air gets thinner, which causes all sorts of funky things to happen at altitude".

That's why I posted a link to Aviatrix's explanation, which covers all that in a most excellent manner, for those of you who wish to explore the inner workings of mach number vs indicated airspeed at altitude.

I wanted to keep my post simple, which is exactly why I took pictures of what the pilot will actually see as a result of all the complicated physics and such.

What you see is that your allowed limiting airspeed decreases :)

Anonymous said...

Thank you for yet another enjoyable and informative entry,
as a flight-sim enthusiast (FS2004) i have seen this phenomena many times at high altitude but never realized quite what was happening

As an aside,, I would also like to thank you for sharing your passion with us
I have been reading all of your blog entries starting from your very first ones
and have now reached mid 2007 i even print them out and read them on the bus sometimes , you have an excellent gift of conveying quite a lot of useful aviation info and yet spiced up by enough of other cool stuff including your interesting outlook on things in general. please keep it up, and i hope you never lose your passion for flying.
Fathi , Palestine.

PlasticPilot said...

I knew an instructor who was doing a funny thing, when training newbies on high-altitude business jets...

On the type he was flying, there were separate circuit breakers for autopilot and autopilot announciator panel. So he flew from the right hand seat, pulled the announciator breaker discretely, so that the trainee in the left seat could not notice it.

The plane was then flown by the autopilot, with no mode anounced. Flying between these two limits at a constant speed is not an easy thing, but autopilots are fine doing it. So the instructor had his hands on the yoke, pretending to fly, and saying things like "look, it's really easy".

Then he passed controls to the trainee, and disengaged the autopilot at the same time (no circuit breaker --> no beeps or alarms). The guy then started to do his best, but was flying in a more shaky way... just because it's not possible to do otherwise when flying manually at such altitudes...

Ah.... old fashioned instruction...