USMCFLYR

Forumname -

No reason to get defensive and call someone "tiger". I asked a question - that is why there is a '?' at the end of that sentence. I asked if you were advocating that. I'm sorry you took that as some sort of challenge.

Back on track. If you are talking about certain situation or airports with special requirements then maybe 10% is not enough. Of course then you are saying that your performance charts (which already take into those factors into account) are more than 10% inaccurate. I generally find my performance charts to be optimistic so I understand what you are saying about the reliability of the information. In any case - MY experience tells me that adding about 10% to numbers will work pretty well in MY aircraft. Your mileage may vary.

USMCFLYR

forumname

Not being defensive. But your post had an accusational tone that I DID advocate it, like I said, not the case.

That's what I was getting at in the FIRST place.

j1b3h0

When I started with my airline, we used to fly Metroliners in to South Lake Tahoe airport. Not only did we have to concern ourselves with takeoff and landing performance but also with things like max balked landing weight, etc,. Tahoe presented an interesting additional problem: One could takeoff to the south - in to the wind, but also toward the mountains which would provide much entertainment event of an engine failure, OR takeoff north toward the lake, but with a tailwind. Multi-engined airliner crews have to show (on paper) that every flight can be conducted with an engine failure at the most critical time, and still meet performance guidelines.
In the summer, we'd fly 19 passengers TO Tahoe - but only 3 FROM Tahoe...

RVSM Certified

Quote:

Originally Posted by rickair7777

Indicated airspeed is generated by air molecules impacting the pitot tube. The faster the true airspeed, the more molecules impact the pitot.

Understood.

Quote:

Originally Posted by rickair7777

However as altitude increases, the air becomes less dense so there are fewer molecules impacting the pitot, so indicated airspeed drops even though true airspeed remains the same.

A high density altitude does the same thing as a high MSL altitude...less dense air.

Ok, I'm good.

Quote:

Originally Posted by rickair7777

Operationally, you normally land and takeoff at the same IAS regardless of density altitude.

Ok, now I'm a bit thrown off by that. That draws a yellow flag in my mind. One the one hand you said that HDA and HMSL do the same thing to the air. Thus, few air molecules interacting with the pitot tube and wing - thus - IAS goes down while [I would assume] ground speed remains the same. This gives the delta in speeds that I mentioned above.

But, then you just said to use the same IAS regardless of DA.

Well, if my DA is sea @ standard day, then use same take-off/landing IAS. Ok, got that. But, if DA is relatively high [enough to significantly impact performance] and if as a result of higher DA the IAS is now lower, then how is it possible to use the "same IAS" in the higher DA? Under HDA or HMSL conditions, IAS drops on the gauge. So, if you use the same "standard" IAS for take-off [for example], then it would seem on the surface that my aircraft would never reach take-off speed.

What am I missing here?

Quote:

Originally Posted by rickair7777

At high density altitude, a higher TAS is required to achieve the needed IAS. This results i higher groundspeed, which means a longer runway is required (for both TO and LDG).

Ok, you've just introduced TAS into the equation, here. TAS is Groundspeed, correct? [should be, no?]

It would seem that a higher TAS would be needed with fewer air molecules entering the pitot due to HDA or HMSL conditions, to arrive at the needed IAS for take-off and/or landing. But, in my mind, I'm still not seeing the correction of what appears to be a contradiction in logic above.

If I use the same IAS regardless of HDA/HMSL conditions AND within those conditions fewer air molecules interact with the pitot and the wing, then it would seem like rotation speed would never be achieved. Help me out here???

Quote:

Originally Posted by rickair7777

As for humidity...air is a mixture of molecules, at a given pressure (altitude) a certain volume of air contains a fixed number of molecules. These molecules can be any of several gasses, including water vapor. Air is mostly made up of oxygen and nitrogen molecules (O2 and N2)...these molecules are actually heavier than a water molecule because H2O has two hydrogen atoms which are the lightest atom on the periodic table. Since the presence of water molecules means less N2 and O2 molecules, that volume of air is going to be less dense. This jeans it will have less effect on the pitot tube and the wing.

Got it, and the chemistry makes sense a well. So, I can visualize the effect of having water vapor inside an air mass in which my aircraft is operating and how that could result in HDA caused by humidity.

I'm ok to go on that one.

Quote:

Originally Posted by rickair7777

Density altitude also effects propulsion...less O2 for the engine to burn, and the prop is just a spinning wing, so it suffers too.

Clearly. I'm ok to go on that one as well.

So, it looks like I'm just stuck on that little bugger of using the same IAS regardless thing. Once I work that out in my mind, this will make more sense.

RVSM Certified

Quote:

Originally Posted by SunDog

...your propeller is producing less "lift in the forward direction" which means less thrust.

Also, your engine is getting less oxygen and so your engine produces less power to turn the prop.

Two very good points it would seem that go to the heart of take-off performance, I would think.

Quote:

Originally Posted by SunDog

Indicated airspeed is measured by the difference between dynamic pressure and static pressure.

I take it that you mean Static Port and Pitot Tube system.

Quote:

Originally Posted by SunDog

...the density is on both sides of that equation and so it cancels out.

You beat me to it - as I was reading it, I thought, "cancels."

Quote:

Originally Posted by SunDog

...therefore, you get the same amount of lift for a particular indicated airspeed regardless of density, but it takes a lot more speed to make that lift at a high density altitude.

Same "relative" lift because of the cancellation mentioned above. I'm good to go on that and even the requirement for more speed prior to lift-off. I'm just not understanding how I can use the same IAS regardless of HDA and/or HMSL conditions.

Quote:

Originally Posted by SunDog

...humidity affects density, but it is a pretty small factor.

What about places near, on or below the equator?

I plan to fly myself and family in to places like Cabo, Miami, Bahamas, Turks & Caicos, or airports like GCM. When I start hitting those runways, do I start to worry about load factors in conjunction with humidity at that point - or can I load and go without too much concern?

Quote:

Originally Posted by SunDog

...the thing to worry about is heat.

Understood.

Quote:

Originally Posted by SunDog

...you don't even need to be full fuel and heavy to have problems due to density altitude. once in an older 172 with only two people on board and half tanks I had to abort takeoff on the big runway at PRC, 5045 feet and 90 degrees.

See, now - this is the kind of stuff that makes me wonder out loud. This is exactly what I'm asking these questions. I don't want to be coming up to the end of a "long runway" finding nose wheel still hanging around having lunch! I want to be off the freakin ground!

How does a pilot handle this stuff? I'm serious about this folks. I've been doing a lot of reading about HDA flying and there are some real nightmare stories out there. I don't want to become a stat, simply because I did something stupid.

What mistakes should I NOT make? Help keep me alive, please! Not to mention my family, friends and future employees and business partners (most especially, my family).

I get really, really, really concerned when I start to see a lack of understanding about Physics being the causality of preventable accidents in General Aviation. There is nothing that scares me about flying, other than my own stupidity and/or ignorance about something that should have been well understood and clearly under my belt.

I'm trying real hard to not develop bad, stupid, dumb habits - very early on in my thought process and flying career. I'm going to literally become a pilot who flies world wide and in many different environments. Thus, I really need to get good at this stuff. It is not like I'm going to be hopping around in a 152 on the occasional weekend around me proverbial "neighborhood." The vast majority of my flights are going to be scheduled business trips, come rain or shine, to various places around the world. I want to be prepared for that with some good HDA flying habits and knowledge.

So, what are the basic, fundamental things that all pilots should AVOID like the plague whenever launching into or departing out of an HDA airport? I'm sure the SJ30 will have excellent HDA performance, but if somebody in an F-18 is concerned about HDA, then I had darn better be concerned about the same in my little SJ, no?

How do I not become a static display behind the yellow tape of the NTSB, because of the pilot syndrome known as "HDA cluelessness?"

RVSM Certified

So, does this mean that all those pilots out there that I'm reading about having problems launching from HDA airports, ditching out at the end of the runway, taking-off and then plowing into something direct in front of slight elevated above the end of the runway, or just taking-off and stalling because they pulled back too far and too soon on the yoke; did not read their performance charts?

If that's the case, then I'm good to go because I'll always read mine!

However, can there really be that many pilots out there who simply "forgot" about the importance of double checking their performance charts before beginning operations at an airport in HDA? If so, is that a training issue that I can get my instructor to focus on a bit more?

I'm mean, based on what I'm reading, something is broken here. Either the student did not get the training at all, or the student got the training and somehow felt that it did not apply to him/her, or somehow the student simply forgot how important it was because every summer I read about the same kinds of accidents.

Heck - they are almost predictable at this point each summer! I just want to know if there might be something in my training that I can tweak or look out for, so this ghost of a problem does not come back to haunt me, either later in my advanced training, or later in my flying career once I've moved on to bigger, higher performing aircraft.

As usual, thanks for the input.

RVSM Certified

You said: more of less in order to get the same. Bingo, that did it! That's what my brain needed, to see a relationship between "less" [molecules] and "more" [impact] resulting in "same" [airspeed]. Nicely done!

Now, I can begin toss this thing around in my head and play with it a little bit. In fact, now that I understand what you guys are talking about here, I think I can restate it a different way:

What you guys are really saying is that the faster ground speed reduces the time interval ("t") between air molecule strikes inside the pitot under HDA conditions. Thus, the reduction in "t" (time) between molecule strikes inside the Pitot, actually fools the ASI into "thinking" that the aircraft is going faster when in fact its actual ground speed could be getting slower [under certain circumstances]. The inverse would also be true. Increase "t" between strikes and the ASI is fooled into thinking the aircraft is getting slower when in fact its actual ground speed could be getting faster [again, under certain circumstances].

Hmmmm. Tossing this thing around in my head even further, presents two additional questions that seem very troubling to say the least:

Question 1: Scenario A

What happens, if during the last quarter of the ground roll during a take-off in HDA conditions, there is a fairly strong gusting headwind that progressively increases non-linearly in relation to the progressively increasing ground speed, forcing the pitot to register a higher IAS on the ASI and the pilot begins to pull back on the yoke or stick too soon, just as the straight-on headwind gust abruptly comes to an end?

Now, in this scenario, the aircraft's ASI showed the attainment of the correct rotation airspeed somewhere after 3/4's of the way into the normal ground-roll [let's say around 82% of the expected ground-roll]. The pilot responds to the ASI by pulling back, then the gusting headwind stops, the ASI drops back to "normal" for this segment of the ground-roll at this HDA, leaving the pilot hanging high and dry with a raised elevator now producing more induced drag slowing the aircraft down even more!

Now, on a "standard day" this would seem to be less serious. But, on a significant HDA day, this seems like it would be enough to really cause some problems for the pilot. Yes/No?


Question 2: Scenario B

A progressive, non-linear gust build-up on the nose of the aircraft during approach to landing [a very stealthy build-up, not a single blast] under HDA conditions. Time interval between strikes in the pitot is reduced, fooling the ASI. Pilot sees this mirage [but does not recognize it as such] on the ASI and either reduces power or pitches up to bleed off some airspeed [now you understand why I asked those Pitch/Power/Trim questions!]. Then all of a sudden, no more headwind gust, ASI drops to "normal" for this HDA condition, leaving pilot with either too low a throttle setting or too high a pitch attitude - or both.

What does the pilot do?

Again, a "standard day" is one thing, but doesn't the stall speed increase as the DA increases due to the need for higher TAS to maintain the same IAS? And, if that is true, is not this scenario a rather dangerous one to have to deal with?

Rememer, the key here in these two scenarios is the progressively [stealthy] increase in the rate at which the headwind is gusting relative to the aircraft's opposing direction ground speed, followed by a rather fast/quick fall off of the gust component after the pilot has already responded to the uptick on the ASI.

As usual, thanks guys for the feedback.

USMCFLYR

Yes to all of your questions. Some were not taught properly, some were taught and forgot and others were taught and just didn't think that the law of physics applied to them. If you are a careful planner and do everything that you are suppose to, every time, then you will be find.
Headwork and decision making can not always be taught. You have to have the WILL to apply it.

USMCFLYR