What the hover charts mean, how to read them, and which one to use.

The “Performance” section of a helicopter’s Pilot Operating Handbook (POH) includes two charts related to hover power: IGE Hover Ceiling vs. Gross Weight and OGE Hover Ceiling vs. Gross Weight. (The exact names of these charts may differ by helicopter manual; these are the names in the Robinson R44 Raven II manual I have handy.) These are perhaps the most important charts in the book — they help you understand the amount of power a helicopter will have under specific operating conditions.

What the Charts Mean

The hover charts enable you to plot your operating conditions to see whether performance is possible. The operating conditions include altitude, gross weight, and temperature — all of which you should be familiar with prior to flight time. You should consult the charts before every flight using the information that applies for that flight.

There are two charts and they work much the same way, although they do provide different data:

• The IGE Hover Chart (below, left) indicates expected performance in a ground effect hover. A note near the chart should get more specific than that. In the example here, the chart is for a 2 foot skid height with full throttle and zero wind.
• The OGE Hover Chart (below, right) indicates expected performance in an out of ground effect hover. Again, a note gets more specific. In this example, the chart is for takeoff power or full throttle with zero wind.

Generally speaking, the IGE chart is for hover operations very close to the ground while the OGE chart is for all other hover operations.

I’ve provided these two charts side by side here, but you can zoom into either one in separate browser window or tab by clicking it.

If you fly a different helicopter, you might want to pull out the charts for it and consult them while reading this. No sense in learning about my helicopter if your helicopter’s charts are handy.

How to Use the Charts

In order to use the charts, you need three pieces of information:

• Altitude of where you’ll be operating. This should be the pressure altitude, but in most instances the elevation above sea level will be close enough.
• Aircraft weight. This is the weight of the aircraft at the time of the operation.
• Temperature. This builds density altitude into the chart so you don’t have to calculate it.

The best way to explain how to read the charts is to provide a few examples.

IGE Hover Chart

Suppose your local airport is at 2500 feet MSL. It’s a pretty warm day and forecast for flight time is 35°C. You’ve done your weight and balance and you know that with you (the pilot), fuel, two passengers, and some added equipment, your weight at takeoff will be 2300 pounds. You want to make sure you have enough performance to do a regular takeoff from a hover, so you consult the IGE Hover Chart. The first thing you notice is that the chart starts at 5,000 feet. Since your airport is below that level, you’re done; the chart only addresses higher elevations so you’re good to go.

Now suppose you’re flying to Flagstaff, AZ, which sits at 7014 feet MSL. It’ll take you an hour to get there and the forecasted temperature there is 30°C for your arrival time. You want to make sure you have enough performance to do a regular landing to a hover and then to the ground, so you consult the IGE Hover Chart. If you burn 16 gallons per hour and each gallon weighs 6 pounds, you’re now nearly 100 pounds lighter or 2200 pounds. You plot the point where the altitude meets the weight. Because you’re still below and to the left of the 30°C temperature line (highlighted in yellow here), you have enough performance. If you follow the 7000 foot line to the 30°C line, you can see that you’d even have enough performance at that temperature and altitude if you were just about max gross weight (2500 pounds).

Now let’s say that when you get to Flagstaff, you’re picking up another passenger and he’s a big guy. You also have to put on some fuel — enough for you to be at max gross weight for takeoff. In addition, takeoff will be at 3 PM when the forecasted temperature is 35°C. Plotting that indicates a problem — the plotted intersection of altitude and weight is above where the 35°C line would be (as estimated in yellow in this example between 30°C and 40°C). This means that under these conditions, the helicopter may not have the performance you need in a ground effect hover.

What could you do about this? A few things:

• Lighten up the aircraft. Take on less fuel (if possible) or leave behind a person or some equipment (good luck with that).
• Wait until the temperature drops. The temperature will likely drop again later in the day. Wait until it drops below 30°C.
• Perform a running takeoff. But is that possible? Because you need less power to fly at speeds in excess of ETL than to hover, it could be. The only way to know for sure is to try. (A serious question here, though: do you really want to take off if you know you don’t have enough performance to even hover in ground effect? Think long and hard about that before doing it.)

OGE Hover Chart

It’s the next day. A photographer wants you to take him and his client around a marina at the lake so he can get video footage of the new dock and some boat races that will be going on. The lake is near Prescott; it sits at 4900 feet and is surrounded by desert mountains. The forecasted temperature in Prescott for the early morning flight is 25°C. You get the passenger and equipment weights and calculate your fuel load for the flight. The total aircraft weight at the time you’ll be on point to start the shoot will be 2200 pounds.

You’ve worked with this videographer before. He likes you to fly very low and slow — usually less than ETL, sometimes only 100 feet off the ground. (You’ve already warned him of the potential danger of this, being sure not to refer to the height velocity diagram as the “deadman’s curve.” But you’re willing to take the risk and he likes flying with you because you’re the only area pilot who will.)

You pull out the POH and turn to the OGE Hover Chart. Why that chart? Well, you know that you’ll be flying at speeds less than ETL. That’ll likely take at least as much power as hovering out of ground effect. You plot the altitude and weight, as shown here. The resulting point is below and to the left of the temperature line (sketched in yellow). You should be good to go.

Now let’s say that the photographer calls an hour before the flight. His client wants to come along, supposedly to provide instructions. You ask how much he weighs and the photographer tells you 240 pounds. A big guy. You recalculate the aircraft weight at the time of the flight adding 250 pounds for the client because you know everyone lies about their weight. Now you’re at 2450 pounds, which is pretty darn close to max gross weight. You plot the points and what do you find? With the client on board, you’d be too heavy to hover out of ground effect.

What can you do? A few things:

• Lighten up the aircraft. Assuming you can’t leave the client behind, you might be able to operate with less fuel on board.
• Wait until the temperature drops. Of course, the photographer picked that flight time for a reason and probably won’t take a time change.
• Don’t drop below ETL when out of ground effect during the flight. If you keep your speed above ETL, you should have plenty of power to fly. But will the photographer accept that?

IGE vs. OGE

In these examples, I used both charts. You might be wondering why I used the OGE chart for a flight that didn’t necessarily require hovering. After all, you reason, if you don’t have to hover, you don’t need a hover chart, right?

Wrong.

As I mentioned earlier, operating at speeds less than ETL — which is about 25 knots in an R44 — requires about as much power as hovering. That’s why you need to consult a hover chart. And since you’ll be 100 feet up or higher, you’re not in ground effect so you need the OGE hover chart.

Let’s look at what could happen in the photography example if you conducted the flight when the OGE hover chart told you hovering wasn’t going to be possible.

You get into position for the photographer to start videoing the new dock. You’re about 100 feet off the lake surface, doing 40 knots. “Slower,” the photographer urges. You drop speed little by little. Finally, you’re at about 15 knots. The helicopter starts to settle. You pull up the collective and either you get a low RPM rotor horn (in a piston helicopter like the R44) or you over-torque or over-temp the engine (in a turbine helicopter). Either way, the settling doesn’t stop. You push the cyclic forward, trying to fly out of the vortex ring state you’ve put yourself into, but you’re now less than 100 feet off the lake. Are you wearing floatation devices? I hope so. You might be going swimming.

Think this doesn’t happen? It does. It even killed people in this accident and this accident — both of which were for aerial photography missions. (And yes, the pilot in the second accident was also impaired.)

Throughout this blog I refer over and over to the OGE hover chart. This is why. It gives you worst case scenario performance for your operating conditions. If the OGE hover chart says you can operate, you can operate at any speed. Indeed, if you consult the OGE hover chart first and it says you’re good to go, you don’t even need to consult the IGE hover chart.

Conclusion

My advice: consult the OGE hover chart before every flight. It gives you a good feeling for the kind of power you’ll have available throughout various phases of a flight, even if airspeed drops below ETL.

This can become vitally important, especially if you’re doing an off-airport takeoff or landing in rough terrain. Think about it. Suppose you’re sitting on the ground near that marina in the last example, with the fatty client on board. Maybe that’s where you picked him up. You pick the helicopter up into a hover; it’s possible but perhaps you need nearly full allowable power to do it. You push the cyclic forward only two or three feet off the ground. What happens as you begin to go through ETL? The helicopter’s nose might dip. If you’re close to the ground, you could hook a skid. Add rising terrain and/or a tailwind and you might find yourself in serious trouble. That’s what happened to a pilot in Easton, WA in 2007 and it killed her and her three passengers.

If the OGE hover chart says you don’t have the performance you’ll need for an OGE hover at your expected flight conditions, at the very least you should consult the IGE hover chart to make sure you can hover in ground effect for takeoff and landing. If you don’t have the performance for that, consider what you can do to increase performance before taking off, even if a run-on takeoff is possible and you’re certain you’ll be able to hover when it’s time to land.

These charts are your true performance guides. Operating when they say you can’t is just plain stupid.

How about preventing accidents by making conditions easier for new pilots?

I stumbled across this brief preliminary accident report in the NTSB database today. Here it is in its entirety:

The pilot who held a commercial pilot certificate with single-engine land airplane ratings was receiving training to obtain a helicopter rating. The pilot had 31 hours of helicopter flight time and the accident occurred on his second solo flight. The pilot reported he was attempting to takeoff from a wet grass area when the accident occurred. He stated that when he increased the collective to lift off, the helicopter began to roll to the right with the right skid still on the ground. He moved the cyclic to the left and the helicopter responded by rolling to the left with the left skid contacting the ground. The pilot then applied right cyclic and the helicopter again rolled to the right. The right skid contacted the ground and the helicopter rolled over onto it’s right side. As a result of the accident, the main rotor blades and the helicopter fuselage were substantially damaged. The pilot reported that there were no preimpact mechanical failures/malfunctions that would have precluded normal operation.

The accident aircraft was a 2003 Robinson R44 Raven II.

Those of us who have soloed in Robinson (or other small) helicopters know that balance varies greatly depending on how many people are on board. We learn to fly helicopters with two people on board: student pilot and instructor. The helicopter is usually quite balanced with two people on board. I don’t have the exact numbers for this particular flight, but using my R44 as a guide and assuming the pilot and instructor were each 200 pounds with 1/2 tanks of fuel, the weight and balance envelopes should look something like this:

If you’ve never seen one of these, let me explain. The longitudinal weight and balance chart shows where the weight is distributed front to back. The pink line represents the main rotor mast. With two 200-pound people on board in the front seat, the center of gravity is forward of the mast. This is pretty normal for an R44. The lateral weight and balance chart, which is what we’re more concerned with in this analysis, shows how the weight is distributed side to side. The 0.00 mark is dead center and, as you can see, the with/without fuel points are very close to the center. In other words, it’s laterally quite balanced.

Now take the instructor out of the left seat. With everything else remaining equal, here’s what the weight and balance envelopes look like:

First of all, the center of gravity shifts aft. That makes a lot of sense. After all, there’s 200 pounds less weight up front. When I fly, this is extremely noticeable, especially at my new, slimmer weight. The helicopter actually lands on the rear of its skids first; on pick up, the front of the skids come off the ground first.

Laterally, there’s also a big change — and again, that’s what we need to focus on here. Without that 200 pounds of weight on the left side of the aircraft, the center of gravity shifts quite a bit to the right. The heavier the pilot is, the more to the right the weight shifts. So although I used 200 pounds in my example, if the pilot was 250 pounds — which is still legal in an R44 — the weight would shift even further to the right (and slightly forward). When the pilot attempted to lift off, the left skid would rise first.

Again, I don’t have the exact numbers. You can play with this using any reasonable numbers you want. It won’t change the conclusion here.

Now read the description of the accident events and put yourself into the pilot’s seat. He’s taking off so he’s lifting the collective slowly, getting the helicopter light on its skids. The left skid comes off the ground first because that side of the aircraft is lighter. This feels to the pilot as if the helicopter is rolling to the right while the right skid is still on the ground.

Stop right there. This is where the terrain comes into the picture.

The pilot is on wet grass. The accident report didn’t say mud so we won’t assume a sticky surface. But it certainly isn’t smooth. We don’t know how long the grass is or whether it’s uniform or has clumps of thick weeds. The question we should be asking is this: was the right skid “stuck” and creating a pivot point? The answer to that question determines the appropriate action:

• If the right skid is indeed “stuck” and creating a pivot point, the correct action to avoid dynamic rollover would be to lower the collective. In other words, abort the pick up.
• If the right skid was not “stuck” or creating a pivot point, the correct action would be to adjust the cyclic to assure there was no lateral movement (as you’d normally do in a pick up) and continue raising the collective.

An experienced pilot would know this. An experienced pilot would have done dozens or hundreds or thousands of solo pickups. He’d have a feel for the aircraft or, at least an idea of what to expect and what to do.

But this wasn’t an experienced pilot. It was a student pilot with only 31 hours of flight time — almost all of which was with a flight instructor beside him — on his second solo flight. So he used the cyclic to stop what he perceived as a roll. He may have been more aggressive than he needed to be, thus causing the helicopter to come down on the left skid. Then another adjustment to the right. The right skid hits that grassy surface and the helicopter rolls. Game over.

So my question is: What the hell was he doing taking off and landing on wet grass?

Flight instructors can reduce the chances of accidents like this by setting up their student pilot solo flights with easy flight conditions. That includes smooth surfaces for takeoff and landing. If this had happened on a nicely paved airport ramp or helipad that was free of surface obstructions like cracks or tie-downs, this accident may not have happened at all. There would be no question about a pivot point because none could exist.

You might also question whether the flight instructor properly taught the concept of dynamic rollover and what to do if it’s suspected on takeoff — lower the collective. And whether the flight instructor made it clear that there should be no lateral movement on pick up or set down. Yes, I know that’s hard for a new pilot to do — especially with only 31 hours — but it’s vitally important, especially on surfaces that could snag a skid.

Which brings me back to my original point: why was he taking off from wet grass?

Fortunately, the pilot was not injured, although the helicopter was substantially damaged. I think this accident makes a good example for teaching about dynamic rollover. With luck, instructors and students will learn from the mistakes here and avoid them in their own training experiences.

Of course, another possibility is the the student pilot simply was not ready to solo.