Author Topic: Commercial Pilots: Question for you (Read 856 times)

Ripsnort · « **on:** July 23, 2003, 07:58:03 AM »

When my wife flew back from Las Vegas last week, one day it was 117 deg. there and one of the flights had to unload 25 passengers in order to take off due to lack of lift in the extreme temperature. Is this a Ground control call, pilot call, or FAA requirement when the temp/humidity reach a certain point? Who makes this call?

Fishu · « **Reply #1 on:** July 23, 2003, 08:05:04 AM »

and I thought the heat increases lift, but thats based on the sail planes.
Guess it might affect to the lift, if the plane is unable to create enough power due to heat readings

Toad · « **Reply #2 on:** July 23, 2003, 08:05:54 AM »

Most modern airlines, it's an automated load planner program call.

That "final paperwork" you're always waiting at the gate to get includes takeoff performance data which factors in temperature, runway altitude, runway length, etc., etc., etc., to give you your max gross weight for takeoff and V1/Vr V2 numbers and the rest.

In the old days, the flight engineer computed this out of books with neat little charts and graphs.

Now the load agent enters the number of pax and the weight of the baggage and the computer printer spits it out.

If conditions make it overweight for takeoff, something is going to change. Usually freight comes off first, then pax and their bags until the numbers are OK. Can't take fuel off for two reasons. Nowadays everybody carries the minimum fuel for economy but also "defueled" fuel can't be used again in an airplane.

Scootter · « **Reply #3 on:** July 23, 2003, 08:10:36 AM »

Quote

Originally posted by Fishu
and I thought the heat increases lift, but thats based on the sail planes.
Guess it might affect to the lift, if the plane is unable to create enough power due to heat readings

When its hot the air molecules are further apart and the air is therefore less dense, the lift is reduced much the same way as it is reduced at you go higher. when its hot and the airfield is high (think Denver) then I gets real bad.

Scootter · « **Reply #4 on:** July 23, 2003, 08:15:05 AM »

Quote

Can't take fuel off for two reasons. Nowadays everybody carries the minimum fuel for economy but also "defueled" fuel can't be used again in an airplane. [/B]

When I was a USAF Firefighter we got to use contamamated fuel for training fires, so we did'ent mind.

ra · « **Reply #5 on:** July 23, 2003, 08:24:30 AM »

The pilot is the one responsible for seeing to it that the plane he flies is within the weight limit for the existing density altitude/runway length. The FAA and ground control have no say over such matters. Even a Cessna 150 has a chart in the operator's handbook showing the amount of runway needed to takeoff under different weight/density altitude combinations. The aircraft manufacturer publishes the chart.

ra

Ripsnort · « **Reply #6 on:** July 23, 2003, 08:39:14 AM »

Thks Toad!

Toad · « **Reply #7 on:** July 23, 2003, 08:52:02 AM »

Yeah, Scootter, I forgot about that use. Valid use, at least. Never minded watching you guys practice!

NP, Rip.

NUKE · « **Reply #8 on:** July 23, 2003, 08:55:03 AM »

at Sky Harbor in Phoenix a few years ago, they shut down the whole airport when it was over 120 degrees, so must not be only the pilot's decision

flakbait · « **Reply #9 on:** July 23, 2003, 09:21:53 AM »

When the temp gets so high that several departing planes have to off-load cargo/pax, the tower makes the call. "Closed due to weather" is a fairly common thing all over the world for airports. Snowdrifts, excess heat, flooding, high winds, low visibility (smaller airports), and such happen every day at one airport or another. Usually it's a temporary thing that causes a lot of delays, but sometimes it's bad enough to shutdown an airport for a few days/weeks.

-----------------------
Flakbait [Delta6]
Delta Six's Flight School
Put the P-61B in Aces High

cpxxx · « **Reply #10 on:** July 23, 2003, 09:31:06 AM »

Ultimately it's the Captain's responsibility but as ever nowdays the system in place ensures several people have a part to play. Uniquely in the US the Flight Despatcher is legally responsible for certain aspects of the flight and it is he or she who produces the final paperwork as Toad described. The Captain makes a decision based on that. The FAA sets rules which must be followed and the airline may or may not have their own stipulations.

It is quite possible that the aircraft concerned would have been able to take off safely at the weight and temperature without offloading passengers but with a reduced safety factor. Perhaps, without the abillity to stop on the runway in the event of an abort or climb after an engine failure or even enroute where an engine failure could result in a descent towards high ground. None of which would be popular with pax!

One of the most difficult exams I did for my Commercial Pilot's licence was called 'Performance A'. Which involved working out all these factors with a series of graphs and charts for a large multi engined jet, actually the Lockheed L1011,Tristar. It was a three hour exam and you needed all that time. (They FAA don't require this exam).

It's all computerised nowdays, thank goodness.

LePaul · « **Reply #11 on:** July 23, 2003, 09:37:21 AM »

So in fancy terms....density altitude?

Went thru this a lot here, especially the hot humid days and two of my pals easily exceed the "average weight" of a passenger.

I remember one time I computed it, I was within the envelope but not by a lot. And the C172 had a smaller engine than the plane I usually rent. That was a pretty long takeoff roll

Big friends make flying a challenge sometimes lol

crowMAW · « **Reply #12 on:** July 23, 2003, 11:10:30 AM »

Quote

Originally posted by Fishu
and I thought the heat increases lift, but thats based on the sail planes.

Even for soaring you can't only look at how hot it is outside to determine good thermal lift conditions. You have to consider the temperature gradient as altitude increases as well. The two gradients that you have to consider are the temperature lapse rate, which is the air temp at different altitudes; and the adiabatic lapse rate, which is the rate of cooling of a parcel of air as it rises from the surface if you assume that it doesn't mix very much with the surrounding air. When the temp lapse rate equals the adiabatic lapse rate you have reached the top of thermal lift.

So if it is really hot and the air temp from the surface to 5000ft is does not change that much, then you don't get much thermal lift.

mjolnir · « **Reply #13 on:** July 24, 2003, 03:37:16 AM »

Yup, LePaul nailed it. DA can be a killer. I got my PPL at the Academy, where the runway sits at 6,500 feet MSL. Throw in a nice summer day when the temperature is in the 70s or so, and suddenly the DA at the field is up around 9,000+. We flew Cessna 172s, and all but the newest ones had a DA limit of 9000. Anything higher than that, we were grounded (the SP models had a limit of 9,500, IIRC).

Actually ran into it first hand by accident though. I took off when the DA was still within limits in an older plane (circa 1972 I think), flew for a while, and was coming back to do some pattern work. Landed the first time, taxied back, and was going to take off again. I had no way of knowing that the DA had gone above limits while I was out flying. Ended up having to abort that takeoff because I wasn't getting 2300 RPMs out of the engine. I'm just glad that I landed that first one and didn't try to go around. Not sure the plane would have been able to do it.

Thorns · « **Reply #14 on:** July 24, 2003, 08:45:58 PM »

I flew out of Colorado Springs one summer, and the temperature was 80 degrees. Now if I remember correctly, I used 10,500 ft. of that 11,000 ft. runway. Pucker Power was on! Took 30 miles of climbing to get over Melody pass. I swore after that I would never fly anything smaller than a turbocharged 180hp around the Rockies. You can pick up any Denver-ColoSprns newspaper, and see a picture of shredded aluminum that some poor sucker didn't know how to calculate density-altitude correctly. Hard way to learn.

Thorns