Guatemala Skies

The density altitude pages have been modified and improved.

The MGGT Density Altitude graph looks better and the calculation should be more accurate. Now they also take humidity into account. The graph shows roughly the last 5 days.

You won’t find High Altitude page anymore. All that information and much more can be pulled from the Density Altitude Airport Query Tool. I was able to modify the page in a way that will allow you to generate a density altitude graph for any airport with available METAR information. All Guatemalan airports with METAR information are pre-configured and can be shown with a single click. But this is no limitation; this should work for any airport reporting weather on the US Aviation Weather webpage. I did include the US highest airport in the quick link as an example.

Hopefully you will find this a useful tool!

The saying goes “When flying from high to low, watch out below” and then there is a similar one for “When flying from hot to cold, watch out below”. The first one is pretty intuitive, but I was having trouble grasping the second one. The reason is simple, pressure altitude is defined as indicated altitude corrected for atmospheric pressure. Nothing about temperature, right? So why would temperature change pressure altitude? Density altitude is a different story. Yes, temperature is a fundamental to calculating density altitude, ok, but pressure altitude? This has circled in my mind for a bit. Thanks for Emilio for giving me the hint to find a solution to this “problem” (at least it works for me :)). Here it goes…

First things first! Temperature does not change pressure altitude directly, it does indirectly though. That explains everything, right? Well, to understand that we have to think about how the altitude instrument functions. It measures air pressure and translates it into altitude. Air pressure can be visualized as weight of the air. At sea level, the air column above you has a specific weight. As you climb you have less air above you, hence less weight and less pressure. The altitude instrument uses these changes to show differences in altitude. Now atmospheric pressure changes with high and low pressure systems also. You can adjust for atmospheric pressure changes in your altitude indicator. Let’s go a step back and think about what is shown in the altitude instrument. It shows altitude, right? And it measures pressure. So how does it calculate the altitude from that? Well, we know that the air pressure decreases with altitude in a certain scale. This scale is based on the International Standard Atmosphere. This is the key! Altitude is not calculated directly, but is translated from pressure using the International Standard Atmosphere. Flying from hot to cold is an experiment where you are changing temperature while leaving all other variables unchanged. The standard atmosphere is a theoretical combination of temperature, pressure, humidity, etc. a single value. If you are lucky, the hot to cold experiment goes through the standard at a certain point. More likely this is not going to happen. See it? The scale is based on a standard day, and the standard day doesn’t really exist. It’s only close! So changes in temperature by definition cannot be standard. Maybe at the beginning its standard, in the end or in the middle, but it can’t be standard all the time. Being much higher or lower than “standard” will make these deviations even more pronounced.

Get it? Temperature changes pressure altitude, because the scale that translates altitude from pressure is based on a standard that doesn’t exist in reality (or rarely). Say the conditions actually reflect exactly the standard atmosphere. You are flying from hot (15 degrees or less) to colder. You changed you standard atmosphere right there. Here in Guatemala, I would not wait for a standard date. 15 degrees Celcius in San Jose? Right!

I can now sleep again :). Please leave a comment!

The trip to Quetzaltenango was long pending. Last time back on December we tried to land in Quetzaltenango the entire valley of Xela was filled with fog. This time around the METAR painted a more promising picture.

After takeoff from la Aurora and flying along the pattern under clouds, we flew a western heading into the blue opening. Clouds over the mountains between Guatemala and Antigua triggered a request to the La Aurora Tower for 10,500 feet inside the control zone and to fly two 360s to reach cruise altitude. At cruise altitude visibility proofed to be unlimited with Tajamulco at sight. The flight was smooth despite an up to 30kts tailwind.

In Quetzaltenango we called in on 118.60 and 127.55 with no response on any of the two frequencies. While we descended into the valley we kept on calling and joined the left downwind leg for runway 23. While flying along the runway we did look down to the runway with no specific observations. We continued the approach and only on short final we noticed a multitude of people, dogs, bicycles, etc. The go-around was interesting. The Quetzaltenango runway is slightly uphill for runway 23 and is at an altitude of just under 8,000 Ft. The go-around is flown at a much lower angle of attack and the climb performance obviously is considerably decreased. Based on the METAR information the density altitude was around 9,000 Ft. No wind was reported, nevertheless at 500 Ft AGL a north wind in our backs could be noted which made the climb feel even less of climb. The optical effect of flying inside a valley (even if it is a pretty large valley) adds to feeling of not being able to climb out. The solution at the end is simple: Fly VX, shallow turns and some patience. Once turned in the wind, it felt better. Definitely an enriching experience in a controlled situation. Back to the “pasos y pedales” on the runway. We decided to go-around and not to land. The chance of a child chasing a ball, a dog crossing the runway or someone with headphones making an unexpected move seamed too high. There were two planes that headed for Quetzaltenango and as far as I can tell, they did land. Maybe the locals are used to planes coming in and will make enough space for landing.

The flight back was beautiful on top of broken clouds most of the times. I heard Guatemala radio inform another pilot headed to Xela about the unofficial Unicom 123.00. Therefore I have updated the frequencies table. One more aspect to point out. … these days flying on top and coming into the La Aurora control zone high, is a useful option. It is no enough to talk to the Guatemala tower and advise about your current altitude, you have to request authorization explicitly if higher than 8,000 Ft.
Another interesting point for these days where flying high allows you to fly on top, when coming into the La Aurora control zone.

A couple of take-away´s and questions:

• Interesting high-altitude almost-landing and go-around
• What are the “official” frequencies for Quetzaltenango?
• Even if unofficial, when is someone responding on this frequencies (not the Unicom obviously)?
• Is it normal for the runway to be crowded and has there never be an incident with landing planes?
• What is the “best” procedure? Low fly-over to make everyone aware that you are coming?
• Remember to ask explicitly permission if entering La Aurora control zone higher than 8,000 Ft. Saying you are at 10,5000 Ft is not enough.

As much as possible, I try to write about Guatemala specific topics. But this article is not bound to Guatemala.

On a recent flight in the copilot seat, the magneto check showed a 300-400 RPM reduction on one side. We were in nowhere´s land, two flight hours away from home base and had commitments. This is why I like to fly with other pilots; it makes you analyze situations from a different angle. I have probably done the run-up checklist about a 100 times so far. It becomes a routine. How serious are you about the checklist? Are you ready to suck up the consequences of an item on the list fails? It appears easy to assure that you would? Really? Back into the Cessna somewhere in Texas, I realized my mind was going over the options. If the magnetos don’t get better after some cleanup work, we would taxi back, then what? Try to get a mechanic? On a Sunday? Drive into town? The guy helping with gas already told us that there were no taxis. Not even sure if we would have gotten public transportation or a hotel. Only one out of two magnetos is affected, right? What are the changes that the other magneto will have similar problems or fail? Wouldn’t it be much better to return the rental plane to its home base? Where have I heard that before? This sounds just like one of the “Never again” stories of the lucky ones that are still around to tell them. The less lucky stories are part of the NTSB accident reports. Doesn’t it almost always start with a not so critical appearing pilot error and then go from there?

So back then I made a note to myself. I promised myself to go through my checklist and decide what the consequences for failed items are. In another occasion I had spotted a recommendation for personal limits that I picked up. It’s fairly simple, write your personal limits (wind speed, direction, visibility, etc.) down and stick to them no matter what. You can only change them after a flight that didn´t meet the limits. Wouldn’t it be good to apply the same concept to the checklist? I will give it a try….

Oh and… we were able to fly back to home base. After running the engine up to 2,000 RPMs and leaning a max for some time, the magneto check came out perfectly fine. Check!

I would love to hear your opinion!

A round-trip video from Guatemala to Coban was added. Watch it on The video Gallery or on the Guatemala Skies Video Channel.

Very interesting visit at the installations of Guatemala´s Air Traffic Control. Every student pilot and flying pilot should have the opportunity to do this visit. Knowing what the controller sees helps tremendously. The aviation community here in Guatemala is relatively small and mostly centralized in the capital. Wouldn´t it be great if Air Traffic Controllers and pilots could meet on a regular basis? This could be a great way to share best practices, common mistakes, recent changes, etc. I would be there every time :).

Greetings to Guatemalan Skies Air Traffic Controllers!

A Bonanza flight to Rio Dulce was added to the Video Gallery.

I have also added a page explaining how get the US verification for your Guatemalan private pilot license.

Have a clear week-end!

On the High Density Altitude Airports page you can follow the trend of Density Altitude for the two highest airports in Guatemala with available METAR information: Huehuetenango and Quetzaltenango.

These graphs are automatically updated and always show the last 99 hours worth of METAR information. Did you know that Quetzaltengo regularly gets a density altitude over over 10,500 feet?

Hello All,

I have created a Guatemala Skies Facebook page where I will be posting when the website has been updated.

Hope this help!
Thanks!
Alex

I realized that the MGGT Wind Trend page wasn´t working correctly. Found the problem and fixed it.

Note that the MGGT Wind Trend and the MGGT Density Altitude sites are not static. The graphs of both sites are generated on-the-fly after pulling data from the Aviation Weather Center. The pages will always show the latest up to date historic data points. This information is not always perfectly formatted. A format that didn´t “make any sense to the program” was the reason for the graph not generating correctly. Again, is fixed now and hope will serve you well.