We started a four day trip today and are just now finishing some SoCal flying before heading east for the red eye. Approaching Sin City, air traffic control had to delay our descent clearance because of traffic crossing beneath us. When our clearance was issued, the controller asked, "Can you guys make it down from there?" That is one of my favorite questions.Is the Pope catholic?
Does a one legged duck swim in circles?
Yeah, we can make it down from here. We are cleared to cross a virtual intersection in the sky west of Lost Wages at 16,000 feet and 287 mph (250 kts). Due to the fact that the clearance came late, we will have to use creative methods of losing altitude. Fi-Fi can shed altitude at an alarming rate. In fact, she can, when approaching from above, blow through assigned altitudes with ease. Her wing spoilers are very effective!
Three thousand pounds of hydraulic pressure shove the spoilers up into the slipstream, and down we go. I turn off the Star Trek mode and push the nose down until the indicated airspeed approaches the barber pole (max allowed for current altitude). The vertical speed indicator (VSI) is buried. I have a mental vertical nav path in my once nimble, but now feeble, brain and am recalculating every few seconds until we merge with the path from above. It goes like this:
A normal, fuel efficient, descent path for a medium size airliner is 2.5 to 3 miles per 1000 feet of altitude. If you are looking at an altitude loss of, say 20,000 feet, then you would need 60 ( 3 x 20 ) miles to lose that altitude. If you must decelerate to meet an assigned speed at the intersection, then you need to allow one (1) mile distance to lose 10 knots of airspeed. Losing 50 knots would take 5 miles. So, we have 60 + 5= 65. That is the base distance to lose 20,000 feet and slow 50 knots. There are a few adjustment factors to keep in mind:
1. Tailwinds increase distance about 2 miles per 10 knots of windspeed. Headwinds decrease distance, but you must be careful. Remember, you have given the controller your assurance that your aircraft will be out of his way at that virtual point in space. How do you know what the windspeed and direction will be during the descent? I use the WAG method honed by years of experience.
2. Anti-icing increases the distance because the engines must increase power to supply the hot air to all of the users, like engine anti-ice and wing anti-ice. You need to allow about 2 miles per 1,000 feet of anti-ice use.
3. Young pilots typically fly faster and need more room to decelerate. The kid I am flying with tonight came from the cockpit of a U.S. Navy fighter eighteen months ago and is still very aggressive... Likes to go fast and burn large amounts of fuel.
In normal conditions, the Star Trek mode does a good job of calculating vertical nav paths and will even try to make a non-normal path. But, of course, it has limits. I remember when the A320s and 319s first arrived on airline property. Some of our old Captains that had been flying Boeings since the Dark Ages decided to take on Sparky before they retired. Most of them made it through training OK, but never really adjusted to the New World Order of avionics. They flew her like an old B-737-100 which is possible, but not very efficient. They would say something like, "Turn that s%#t off and fly this thing!" Yes sir, Boss.
As we merge with the vertical path in my brain, I raise the nose and start the deceleration segment. The spoilers remain up until the airspeed begins a downward trend toward 287 mph (250 kts). The Star Trek path looks identical to my mental path... I return control to the flight management computers, and stow the spoilers. Fi-Fi spears the intersection on altitude and airspeed, and then banks 30 degrees left to intercept the outbound course. We are rolling in on Sin City.
It is a beautiful, smooth night. As we clear the terrain west of Lost Wages, the city comes into full view. Wow! It is bright. Things will happen fast for the next 60 minutes. We've got to get this beast on the ground, re-fuel, re-load, make a taco run, and get airborne for points east.
Position: 25 miles west of Las Vegas
Altitude: 14,000 feet
Airspeed: 287 mph (250 kts)
15 comments:
Excellent post... the split second mental math is yet another challenging part of flying I don't often give much thought. I'd definately be reaching for a calculator to figure it all out.
A question: How far from the airport and at what altitude do you put down the landing gear?
Cool Dave, this means your about 3,000 feet above the mountians there...would like to experience one of those bagdad approaches from the front seats..
Craig in MCO..
joel- normal gear down point is about 5 miles from threshold. However, if you are using the gear for drag to help lose speed and altitude, 5 to 20 miles is normal.
Great description - as always. Just like being there. The Deisel Nine was also fun in these "can you get down" events because along with the speed-brakes, we could take "slats only" at a high speed (280kts I think it was...). Then watch out below...
Thanks for lovely photo of LAS.
That is almost too much math!! Haha!!
I understand that, during a normal descent, ATC likes to see you reduce your vertical speed during the last 1,000 ft., as a hint that you know where you are and you're not about to bust your level. I assume that this becomes even more important when you're descending with the VSI pinned at max....
I was down in the very southern states only once. And the tacos were great. I think I'd be making a run for them too. Good post to see inside the mind of an airline pilot.
I think its hilarious that you call her "FI-FI"....hahaha - Thats kinda like a dog name - "The A320, a well behaved dog"...LOL Excellent writing, Dave.
Everytime I see Dave refer to FiFi I visualize the aircraft with a beret and B*tchin' Betty with a french accent.
Dave - fantastic blog, first of all. Second, what's the fastest descent ATC has ever asked you to perform? Did you do it?
courtney- atc will not ask a pilot to do something that obviously can't be done. I have done some very steep descents with Fi-Fi that could not be done in other types, though. If I am not sure I can make it, I will tell atc they can have either the altitude or the airspeed but not both at the intersection; then they will give us relief from one of the parameters.
Dave, thanks for the nice post and picture! Is this in preparation for a touchdown on one of the 25s? Just asking because the picture really looks like it is a good idea to get a seat on the left side when flying into LAS from anywhere on the west coast, which I will be doing later this year. Flightaware.com looks like they mostly take you over the city in this general direction when you come from the west because you'd otherwise have to be going east of dreamland and that's a big detour? Is it mostly the 25s or sometimes one of the 1s, too? Sometimes, with wind from the east, the 7s as well, giving you a very close look at the strip?
zb- we were coming from california eastbound over the redrocks west of the airport; sort of an extended left down wind leg for 25left, which is the primary landing runway, wind permitting.
"my once nimble, but now feeble, brain..." You've got to be joking!
Thanks for another great time with you on the flightdeck, Dave, and photo of approaching Las Vegas. We'll be visiting those red rocks later this year.
Keep safe.
Some one rings the bell, I wait till I am 60 feet from the stop and press the brake pedal down and roll to a stop, press the red button and the door opens. Much simpler than your stopping method but your's sounds like more fun.
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