Ask The Pilot

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4780 feet - density altitude

Thank you, if density altitude = sea level what would Temp and air pressure be?

Thats nearly 1.5km!! Nearly that of "mile high" DEN!! Would DA at DEN be lower than its 1.6km altitude on a cold winters day?

 

[petercr]

What would the apparent altitude be if a runway is 100ft above sea level with 993hPa and 49 deg celcius?

Thinking about EK521?

 

[flychrisfly]

Does the APU have its own fuel tank? If not, as I suspect, how is it fuelled?

 

[Boris spatsky]

Thank you, if density altitude = sea level what would Temp and air pressure be?

1013 Mb and 15 degrees C is a standard day at sea level (known as an ISA day).

 

[Boris spatsky]

Thats nearly 1.5km!! Nearly that of "mile high" DEN!! Would DA at DEN be lower than its 1.6km altitude on a cold winters day?

Denver is about 5430 ft. On a standard day it would be 4C (based on 2 degrees per 1000 ft).

Yes, the DA would generally be lower on a cold day. Temperature has a 4 fold effect compared to barometric pressure on density altitude.

If the baro was 1013, and the temp -6C, the DA would be roughly 1200 ft lower than airfield elevation, so about 4230 ft.

Density altitude in layman's terms determines how an aircraft performs. So a high density altitude means the aircraft performs worse than at sea level (as if it were at that altitude).

 

[Boris spatsky]

Does the APU have its own fuel tank? If not, as I suspect, how is it fuelled?

I can't speak for all manufacturers but most feed the APU from one of the main tanks.

 

[Boris spatsky]

Businesses world wide are sweating on assets extending the shelf life, are we seeing this with airlines and will this result in problems in the long term?

I don't see it as a major problem in high capacity passenger operations because the airworthiness standards are high (generally).

They are much less stringent in charter and private operations - that is where i see the issue arising IMHO with operators extending the life of the aircraft.

Plenty of military airframes have lasted exceptionally long periods (B52 anyone??) - it just costs exponentially more to keep them going the older they get.

 

[jb747]

Curious about cameras in the coughpit, are they allowed or is permission required from airline?

In some youtube videos, they cover the call sign plaque is there any real reason for this?

I expect that there are as many different rules as there are airlines. As we can't take our family in the coughpit any more, for most of us, the images that we get will be all we can show them of our jobs. As long as it doesn't affect the operation, I see no reason why cameras shouldn't be there.

 

[andmiz]

I expect that there are as many different rules as there are airlines. As we can't take our family in the coughpit any more, for most of us, the images that we get will be all we can show them of our jobs. As long as it doesn't affect the operation, I see no reason why they shouldn't be there.

And at the other end of the spectrum, we can allow our family in the coughpit under specific criteria/rules, but don't allow cameras in the coughpit.

 

[mjt57]

I can't speak for all manufacturers but most feed the APU from one of the main tanks.

Wasn't that one of the problems that QF32 faced? They had a full rear tank, so they started the APU to try and draw it down but they couldn't get its generator connected, so it sat here idling.

 

[mjt57]

Hey JB (and Boris if you drive heavies), how'd you go in this weather?

https://www.facebook.com/TheAcquaPlanet/videos/1631095577182053/

 

[Boris spatsky]

Hey JB (and Boris if you drive heavies), how'd you go in this weather?

https://www.facebook.com/TheAcquaPlanet/videos/1631095577182053/

The video is a 737 (so, not a heavy). Looks like a normal crosswind takeoff within aircraft limits (guessing about 30 knots-ish).

 

[jb747]

Wasn't that one of the problems that QF32 faced? They had a full rear tank, so they started the APU to try and draw it down but they couldn't get its generator connected, so it sat here idling.

Ah, the myth of QF32. It just goes on giving.

The APU does not get any fuel from the tail (trim) tank.

I hadn't thought about it before, but they'd have tried to light up the APU in flight. One generator can be brought on line in flight, and the other after landing. But, if the system were not able to incorporate the power then it would not do so. Power distribution is automatic. I don't know whether it started or not. The fuel transfers were very compromised, so there is no guarantee that it would have had any fuel source. If I remember, I'll ask next time I see one of the guys.

If neither generator was on line, then it could still provide bleed air to the packs. Whether activating any of these systems was actually done, or would have been sensible, is another topic entirely.

 

[flychrisfly]

One generator can be brought on line in flight, and the other after landing.

So the 380 has 2 APUs? What's the difference and why and ????

Thanks

 

[petercr]

Slightly off topic but interesting and JB has probably seen this IRL...
Airbus A380: Pratt & Whitney Canada (P&WC) PW 980A APU. Pratt...

 

[mjt57]

Ah, the myth of QF32. It just goes on giving.

The APU does not get any fuel from the tail (trim) tank.

OK, I'll have to go back to RdC's book and have another read.

 

[mjt57]

The video is a 737 (so, not a heavy). .

I said that as I wasn't sure what you fly, either heavies or not-so-heavies...

 

[jb747]

So the 380 has 2 APUs? What's the difference and why and ????

It has two APU generators, not APUs. They are identical. But, the system only allows one to be connected to the busses in flight. Any of the engine driven generators can provide all of the load needed for the real aircraft systems. There aren't many scenarios that would remove more than one from play...and for those days, you can use that APU generator. Unlike the big twins, the APU isn't available in the full flight regime....your redundancy comes from the fact that each engine is fitted with one.

The RAT (ram air turbine) is there for the day that every generator dies.

 

[Awesom Andy]

While on the topic of the APU, I was on a flight recently that had the APU u/s. Apart from the more long winded procedures to start the engines, and pax complaining on the lack of IFE at the gate, what else would be affected?

 

[jb747]

While on the topic of the APU, I was on a flight recently that had the APU u/s. Apart from the more long winded procedures to start the engines, and pax complaining on the lack of IFE at the gate, what else would be affected?

Starting is straightforward enough. You just need to get at least one (and preferably two) under way whilst still at the gate. The high pressure air is supplied by external starter units. At most terminals, external air and electrics are used, not the APU. Once the engines are running the APU is normally shut down anyway, so you don't actually lose anything from that point. Cross bleed starts can be done with the running engine(s) at idle.

Sometimes we run the air-conditioning on the APU for take off (to get extra performance) and we won't be able to do that...which could mean using a longer runway. If we were to lose an engine, we'd not be able to regain CAT 3B capability, but that's rarely useful anyway.