Tow Plane Flies Too Slow – Glider Heavy with Water Ballast

A few weeks ago I had a scary situation when towing fully ballasted behind a Piper Pawnee.  I had my camera running and published a short instructional video about the incident. I hope that it helps others avoid similar situations in the future.  Here is the video:

The comments indicate that these incidents are relatively common and occur most frequently when the tow pilot is used to pulling very light gliders that only require a low tow speed. A clear and deliberate briefing of the tow pilot before the flight is essential. You can also find a similar incident in one of Bruno Vassel’s videos.

Moreover, Dave Nadler gave a safety briefing on this very topic a few years ago at a US Soaring contest.  In it he explains that the glider’s stall speed on tow is actually higher than it is in free flight.  And why the problem is exacerbated behind short-winged tow planes such as Pawnees.  Please take a few minutes to also watch Dave’s video.  You can find it here.

A few good questions came up in this context, which I would like to address here.

Why did you not release immediately?  Would you release if a similar situation were to happen again?

I’ve spent a lot of time thinking about this question.  At the moment I was too afraid that the glider might stall and drop hard to the ground if I pulled the release.

However, perhaps the safest response for everyone involved would have been the following:

    • Rather than trying to climb with the tow plane, I should have stayed in ground effect while communicating the request for a greater airspeed.  Staying in ground effect protects the glider from a stall because of lower induced drag.  It also reduces the consequences of a possible drop to the ground because of the very low altitude.
    • Hopefully the tow pilot would react before the glider reaches the “low tow” position.  (We don’t practice low tow in the US except when practicing “boxing the wake”. In some countries the low tow position is used more routinely for towing because it protects the tow pilot from a glider pulling the tow plane’s tail up and forcing the tow plane into the ground.)
    • Once the glider has reached the low tow position, and the tow plane continues to climb at an insufficient air speed, pull the release.  (Hanging on for longer would put the tow plane in danger.) Immediately release stick pressure and simultaneously move the flaps into landing configuration.  Land straight ahead and only extend the spoilers once the glider is on the ground.

This plan only works if there is enough runway left for landing straight ahead. In Boulder we definitely have enough space available.

My main concern with this strategy is whether staying in ground effect until the low tow position is reached could be pulling the tow plane’s tail down to the point where it becomes difficult for the tow plane to get its nose down and pick up speed.  However, considering that low tow is a normal tow position in some countries, I assume it should not be a problem, at least unless the tow plane itself is close to stall speed.

If you have additional thoughts on this subject, please add them in the comments below. I am especially interested to hear from glider pilots who normally fly in low tow position.

PS: Dave Nadler’s explanation is very compelling and illustrates that a glider’s stall speed is actually higher on tow than it is in free flight.  This gives me more confidence to release immediately in the future because releasing is unlikely to lead to an immediate stall because the stall speed comes back down.  So my plan is to release, release stick pressure while simultaneously moving the flaps into landing configuration, then land straight ahead with the spoilers closed (and only open them as soon as the glider is on the ground.)

“Faster, Faster, Faster” is not clear communication. It would be better to say “Up 5” or “Up 10” or “Up 15” or “Up 20” (which is what I needed).

Maybe.  The thing with urgent communications is that the best one to use is the one that is instantly understood.  I readily admit that “Faster!” is not precise and may sound unprofessional but I think it conveyed a sense of urgency that “Up 20” may not have.  I am also not certain that the tow pilot would have instantly understood and responded to “Up 20”.  The lesson here, at least in my mind, is that tow pilot and glider pilot should make communications part of the briefing so both parties are in synch on what language to use.  I’m curious what you think the best protocol should be when you need an instant reaction.

The communication should have included the call sign of the tow plane.

Yes.  In principle this is certainly true.  A clear and non-ambiguous communication would be something like “X-Ray Yankee Zulu, 10 more knots.” (And then ask for another 5-10 knots if necessary.)  However, in the stress of the situation I did not remember the call sign even though I had said it myself only two minutes earlier.  (We have 5 different tow planes at the field.  I’ll definitely try to remember it better the next time.)

Also, if you watch the video you’ll notice that I had to communicate in a fraction of a second while also considering all the other choices such as whether or not to release.  What mattered more than anything was a prompt ]reaction by the tow pilot. (Basically to level off or push the nose down and pick up speed).  Just saying the abbreviated three digit call sign takes an entire second and saying “X-Ray Yankee Zulu, 10 more knots” would have taken about two seconds which I didn’t really think I had.

What will you do differently going forward?

The main thing is to adjust my briefing to avoid such situations in the first place.  I don’t always know what the air speed units are in the specific tow plane ahead of me.  Therefore, I am now requesting a minimum tow speed in knots AND MPH.  E.g., I’m now saying “Towplane XYZ, behind you is glider Victor One, Fully Ballasted, Minimum Tow Speed 70 knots or 80 MPH.”

You can find a collection of my favorite soaring safety articles on this page.

16 Replies to “Tow Plane Flies Too Slow – Glider Heavy with Water Ballast”

  1. Speeds, signals, and safety. Surely comments pointed to the regulatory prohibition on aerotowing unless “[t]he pilots of the towing aircraft and the glider or unpowered ultralight vehicle have agreed upon a general course of action, including takeoff and release signals, airspeeds, and emergency procedures for each pilot.” (14CFR91.309(a)(5))

    1. Hi Lon, thanks for the comment and the reference to the regs. I would say that the briefing met all the regulatory requirements. We are all trained on the standard tow signals, we had agreed the airspeed as 70 knots, and we are also trained on standard emergency procedures that cover what to do in the event of a rope brake or in case of loss of tow plane engine power. We had even agreed that I would indicate speed changes if needed via radio in case we would be somewhat slow or fast. The standard tow signal for increasing speed is for the glider to rock the wings. Attempting this at a low altitude and with a barely controllable glider near stall speed would have been a terrible idea. And it would also have been far too slow to work. Standard procedures are great because everyone used the same ones – except when they are not.
      Obviously we had not specifically discussed the possibility that the tow pilot might make a unit conversion mistake. Had this been on our minds, the mistake would not have happened in the first place. However, I would agree that I should have a contingency plan for this situation. I now do (as mentioned in my post), but I am not 100% certain if it really is the right one. If you have specific recommendations for how to avoid such situations (in addition to those already mentioned) that would be very helpful. Thanks! Clemens

  2. I’d like to know your view of what would have happened had you stalled while still attached to the tow rope.

    PS Thanks to you and tow pilot for publishing this.

    1. Hi Eric, good question! I don’t really know and definitely never want to find out! You’d certainly have to release immediately if the glider were to stall.

  3. A not uncommon occurrence – in my ten years of flying a Discus 2b, I had a couple a year. A slow tow would typically result in poor roll response and insufficient elevator that was unsettling at best. I released a couple of times when the tow plane would not or could not accelerate. One issue appears to be lousy accuracy of air speed indicators on tow planes.

    It’s absolutely essential to preface any speed change request with the tow plane’s call. At a contest once, one pilot called “ tow plane slow down 10” when there were seven in the air!

    1. Hi Mike, thanks for sharing your experience. I would have used the towplane’s call sign had I remembered it. But in the stress of the situation it slipped from my mind. It’s a good practice for call plane ID’s to be labeled on the flaps in big letters (instead of just on the fuselage where it is not visible) so they can be read from the glider.

  4. Clem,
    One thing I learned from Jim Lee is to have a “come to Jesus ” chat with the tow pilot. You tell them that if they start climbing out without sufficient airspeed, YOU just killed me! That seems to get their attention.
    Dave Leonard taught me that if the tow plane gets too slow on tow, add flap. It will help you balloon up to the altitude of the tow plane and lower your stall speed. Now when just taking off, realize more flap increases drag. I have not tried that trick below 200 feet.
    Clay Thomas
    Glider pilot and tow pilot

    1. Hi Clay, thanks for your comments.
      Re: the first one: the tow pilot and I debriefed in detail and I have no doubt that he’ll never do this again – just as I will never again brief simply in knots – I regard this mainly as my mistake. I should not leave the unit conversion to the tow pilot to be done on the fly (literally). The problem I’m concerned about is not so much how to prevent the same mistake by the same person but how to prevent other pilots making the same mistake – even once. (Even in Boulder we have 20-30 different tow pilots so next time I fly it’ll most likely be someone else flying the tow plane…) The issue is that first time can kill – and not just the glider pilot, the tow pilot is almost equally vulnerable as the glider might just pull the towplane down with it… So how can we help ensure that that there is no first time for any pilot? That’s a difficult challenge but it must start with awareness. Just like a glider pilot can easily kill a tow pilot by ballooning behind the tow plane (and far too many tow pilots have died this way), it is also possible that a tow pilot could kill the glider pilot (and themselves) by flying too slowly. It does not seem to have occurred as often as the reverse but that is probably mainly due to the fact that only a small fraction of all tows involve heavily ballasted gliders. I think many tow pilots are not aware of the risk, in part because they rarely encounter it and in part because they can’t feel it themselves. I also think that glider pilots may be insufficiently aware about the risk – at least until they experience it first hand. But then it may be too late. So awareness is key to reduce the number of “first times”.
      Re: flaps, yes, I regularly adjust my flap positions on tow and I am very familiar with the effect and the changes to the stick position that they require to stay nicely in position behind the tug. I usually have flaps in -1 during the start of the ground roll, 0 at about 30 knots, then +1 during lift off, then sometimes adjusting to 0 after the targeted tow speed is reached (especially when flying dry). I have also towed at flaps +2 and I probably should have moved flaps to +2 on this tow but I was preoccupied with the decision whether to release so I don’t think I did. One downside with strongly positive flaps is of course the reduced aileron control – especially at low speeds. I’d consider increasing flaps more as an emergency response than as risk mitigation strategy. The key goal must be to prevent these situations in the first place, so that brings me back to awareness… which is why I decided to publish the video.
      Thanks again for your thoughts!

  5. Thanks, Clemens, for another great blog post and YouTube video! Your documented flights, presentation style, analytical approach, and most of all, your willingness to share the best and worst moments do so much to advance the safe practice and enjoyment of soaring.

    Might I suggest that if tow plane’s ASI is only in mph, then say only one speed (e.g., 80 mph) during your takeoff briefing with the tow pilot. Maybe repeat that number once or twice. Mention no other tow speed. In the (hopefully unlikely) event that the tow pilot fixates on some other speed, at least you didn’t inadvertently plant that bad number in his head.

  6. … not saying that you planted any bad number on the day in question. That apparently happened organically in the tow pilot’s brain , which just makes the case for briefing/repeating only one tow speed when practical in order to better impress the correct speed on all concerned.

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