why DME??

I_WANNA_BE_ATP

I_WANNA_BE_ATP

New Member
Can anyone tell me why the FAA makes DME a requirement for flight s above 24,000 feet?

Also, does anyone actually ever use VFR on top???

Just some questions that popped into my mind during my instrument ground. The instructor wasn't sure why DME is needed. His best thought was if you are not in radar coverage and ATC wants to know your location. He also said VFR on top is useless. I tend to agree.
 
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He also said VFR on top is useless. I tend to agree.

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And if there's ice in the clouds and you're in a C172? Seems pretty worthwhile then....
 
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The instructor wasn't sure why DME is needed. His best thought was if you are not in radar coverage and ATC wants to know your location. He also said VFR on top is useless. I tend to agree.

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good instructor
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The instructor wasn't sure why DME is needed. His best thought was if you are not in radar coverage and ATC wants to know your location. He also said VFR on top is useless. I tend to agree.

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good instructor
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That's a pretty harsh comment. Why would 99% of instructors who don't fly in the flight levels know the answer to this question?

The only thing I can think of is for navigation purposes. DME is good for 199nm from the station (supposedly). But what if I am over the Atlantic at FL360...not much good there?

VFR-on-top is not useless. I have used it before. ATC tried to route me all over the place in Texas. I was VMC (on top of a layer) and wanted to go direct. Told them I wanted to cancel IFR..."squawk VFR, frequency change approved, have a good flight".
 
actually, you're right. i didn't mean to get the part about DME in there, just the VFR-on-top. VFR-on-top isn't useless.

btw, if you cancelled IFR, that doesn't qualify as VFR-on-top, it qualifies as VFR. if yu're VFR-on-top, you're still on an IFR flight plan and you're still in touch with ATC squawking an IFR code. there's a big difference between VFR-on-top and getting a clearance to pop above some clouds and then cancelling.
 
1. VFR-on-top is not useless (we both agree)

2. Don't criticize instructors for not knowing answers that have nothing to do with there particular type of flying.
 
Actually, my instructor used to work for a major airline and is now retired with thousands of hours. I guess I should have included that he suggested that you should fly in the IFR system as much as possible.
 
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1. VFR-on-top is not useless (we both agree)

2. Don't criticize instructors for not knowing answers that have nothing to do with there particular type of flying.

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You shouldn't criticize an instructor for not knowing an answer, but you should criticize them for pretending to know something, or just blabbing about something they don't know about.

Instead, an instructor should say, "I don't know the answer to that. Let me do some research and get back to you."

My .02....

G
 
A great use of VFR on top is when you are wanting to cruise below the MEA on a route that is extremely high, like for example the V484 out of SLC to BOI has a MEA of 16000 but you don't need that for terrain clearance, only reception. So you request VFR on top and cruise at 11,500 and stay below the require O2 altitude.

An additional place where it makes a tremendous amount of sense is coming into a busy airport and starting your descent when you want to rather than when you are told to. When arriving into SLC on a VFR on top clearance you can put in a VNAV profile that will get you out of high altitude (again 02 requirement), increase your forward groundspeed, and make your routing and handling much easier than arriving via a STAR.

In truth the VFR on top clearance is really helpful so long as you know how to use it. Use it as though you don't really want to cancel your IFR clearance but you want to provide your own terrain clearance (and traffic of course).
 
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