Can Marine One Fly in the Rain?

Why, exactly, did Donald Trump not join Emmanuel Macron, Angela Merkel, and Justin Trudeau at Saturday’s commemoration of the 100th anniversary of the original Armistice Day? I don’t know, and I don’t think anyone outside the White House does at this point.

What I do know is that one hypothesis that has shown up in many stories about his no-show—that Marine One, the presidential helicopter, “can’t fly” in the rain—doesn’t make sense.

As you’re looking for explanations, you can dismiss this one. Helicopters can fly just fine in the rain, and in conditions way worse than prevailed in Paris on November 10.

First, about helicopters and weather. (What follows is based on my having held an instrument rating as an airplane pilot for the past 20 years, and having worked in the Carter-era White House and occasionally having been aboard the Marine One of that era.)

There is nothing special about the rain-worthiness of the helicopter any president normally uses. In principle, any helicopter can fly in clouds or rain. The complications would be:

Icing: This is one of the big weather-related perils of flying. (The other is thunderstorms.) If (a) an aircraft is inside the clouds, and (b) the temperature is at freezing or below (down to about -15 or -20 degrees Centigrade, when it becomes so cold that the water behaves differently), there’s a risk of icing: ice piling up on the wings, control surfaces, etc. This changes the shape of the airfoils, and it essentially makes a plane unable to fly. This was part of the story of the commuter plane that crashed going into Buffalo a few years ago. I did a long illustrated post about what icing looks like, and how it kills, back in 2011. Other posts are here and here.

But this only happens if a plane or helicopter is actually in the clouds (“visible moisture” is one of the requirements), and for a helicopter that, in turn, would mean a ceiling so low that a helicopter could not fly beneath it, clear of the clouds. Or it could occur if the temperature profile were such that you get “freezing rain”—rain falling through a supercooled atmospheric layer and being at or below 0 degrees Centigrade when it hits, thus instantly freezing on whatever surface it touches.

Extremely low ceiling: If the ground were essentially fog-covered, so the pilot couldn’t judge when he was about to touch down, that could be too dangerous to fly in. Think London pea-soup fog, or a bad day in Beijing.

So: Helicopters cannot safely fly inside the clouds when it’s below freezing, and they can’t safely or prudently land when there is dense fog or other very low-ceiling circumstances. And they cannot fly safely if it’s extremely windy and gusty—which can make it dangerous to land.

Otherwise, just about any helicopter can fly in rain, bad weather, etc.

From photos of Paris and the commemoration site Saturday, it didn’t appear that the ceiling was so low (or the temperature so cold) that icing would be a real factor. So just now I have dug up the archived “metars”—the aviation-related weather reports—for Saturday at Orly Airport in Paris, the closest reporting station to where Trump was. Here’s the raw data for Saturday’s Orly metars, at half-hour intervals essentially from dawn to dusk:

SA	10/11/2018 17:30->	METAR LFPO 101730Z 20010KT 9999 BKN012 12/10 Q1002 NOSIG=
SA	10/11/2018 17:00->	METAR LFPO 101700Z 20011KT 160V220 9999 BKN012 13/11 Q1002            NOSIG=
SA	10/11/2018 16:30->	METAR LFPO 101630Z 20010KT 9999 BKN011 13/11 Q1002 NOSIG=
SA	10/11/2018 16:00->	METAR LFPO 101600Z 20010KT 9999 BKN011 13/11 Q1002 NOSIG=
SA	10/11/2018 15:30->	METAR LFPO 101530Z 23007KT 190V260 9999 BKN010 13/11 Q1001            NOSIG=
SA	10/11/2018 15:00->	METAR LFPO 101500Z 20010KT 9999 BKN010 13/11 Q1001 NOSIG=
SA	10/11/2018 14:30->	METAR LFPO 101430Z 20010KT 170V230 7000 -RA BKN010 13/11            Q1001 TEMPO 2500 RA BKN006=
SA	10/11/2018 14:00->	METAR LFPO 101400Z NIL=
SA	10/11/2018 13:30->	METAR LFPO 101330Z 21010KT 170V250 9999 -RA BKN011 12/11            Q1001 TEMPO 2500 RA BKN006=
SA	10/11/2018 13:00->	METAR LFPO 101300Z 21009KT 180V250 9999 -RA BKN010 12/11            Q1001 TEMPO 2500 RA BKN006=
SA	10/11/2018 12:30->	METAR LFPO 101230Z 25010KT 7000 -RA BKN010 12/11 Q1001 TEMPO            2500 RA BKN006=
SA	10/11/2018 12:00->	METAR LFPO 101200Z 19012KT 9999 -RA BKN009 12/11 Q1001 TEMPO            2500 RA BKN006=
SA	10/11/2018 11:30->	METAR LFPO 101130Z 20011KT 3000 -RA BR BKN006 OVC013 12/11            Q1001 TEMPO 2500 RA BKN006=
SA	10/11/2018 11:00->	METAR LFPO 101100Z 19011KT 2700 -RA BR OVC006 11/11 Q1002            NOSIG=
SA	10/11/2018 10:30->	METAR LFPO 101030Z NIL=
SA	10/11/2018 10:00->	METAR LFPO 101000Z 18011KT 6000 RA BKN008 11/10 Q1002 NOSIG=
SA	10/11/2018 09:30->	METAR LFPO 100930Z 19011KT 7000 RA BKN010 11/10 Q1002 TEMPO            4000 RA BKN009=
SA	10/11/2018 09:00->	METAR LFPO 100900Z 18010KT 9999 -RA BKN010 11/10 Q1002 TEMPO            4000 RA BKN009=

I am not going to bother to decode this all. (Being able to read metars is part of ground school in the learning-to-fly process.) But here are the essentials:

• On Saturday morning, the weather in Paris was rainy and overcast—bad weather, but not any exceptional aeronautical challenge. The worst conditions during the day in Paris were at noon, when there was an overcast ceiling of 600 feet. (“101100Z 19011KT 2700 -RA BR OVC006.” The -RA means “light rain.” The BR means “mist,” and the mnemonic for remembering it is “Baby Rain.”) As a benchmark: To get an instrument rating, whether in an airplane or a helicopter, you have to show the ability to fly an approach “to minimums,” which (depending on the airport and the approach system) can be as low as 200 to 300 feet. Still, on Saturday morning, a helicopter trip from Paris would probably have meant spending part of the time in the clouds.
• The temperature in Paris through the morning was 11 to 12 degrees Centigrade, or the low 50s Fahrenheit. That is not very cold. The normal “lapse rate” for air temperature is about 2 degrees Centigrade colder for each thousand feet you go up. In normal circumstances, that would mean the freezing level was at an elevation of around 6,000 feet. (To spell it out: 12 degrees Centigrade at ground level, minus 2 for each thousand feet, means that you reach 0 degrees Centigrade around 6,000 feet up.) So at an altitude of 3,000 or 4,000 feet, this would not be an icing-peril scenario.
• It was not very windy. Through the morning, the wind was reported at 11 or 12 knots—not enough to worry about.

Of course, safety considerations are different when a president is traveling. The pilots and maintenance practices of Marine One are presumably the best that can be found, but the play-it-safe factor when carrying a president has to be larger than for other missions. So who knows whether some aviation official really said: Sorry, this is no-go.

But precisely because of those cautions and complexities, any known-universe past presidential travel plan would have a bad-weather option, or several of those, already lined up. This is the way it has worked in any White House I’ve been aware of.

Why didn’t an American president go to a once-in-a-lifetime ceremony? Might it have been a still-undisclosed security threat? Something else that Donald Trump had to do?

I don’t know. I do know that whatever the obstacle was, it wasn’t that “helicopters can’t fly in the clouds and rain.”