diff --git a/libraries/GCS_MAVLink/GCS.cpp b/libraries/GCS_MAVLink/GCS.cpp
index 93c549403b..02d7f71ef4 100644
--- a/libraries/GCS_MAVLink/GCS.cpp
+++ b/libraries/GCS_MAVLink/GCS.cpp
@@ -256,3 +256,8 @@ bool GCS::out_of_time() const
 
     return true;
 }
+
+void gcs_out_of_space_to_send_count(mavlink_channel_t chan)
+{
+    gcs().chan(chan)->out_of_space_to_send();
+}
diff --git a/libraries/GCS_MAVLink/GCS.h b/libraries/GCS_MAVLink/GCS.h
index 48f759d0e0..898d7ab62b 100644
--- a/libraries/GCS_MAVLink/GCS.h
+++ b/libraries/GCS_MAVLink/GCS.h
@@ -29,6 +29,8 @@
 
 // macros used to determine if a message will fit in the space available.
 
+void gcs_out_of_space_to_send_count(mavlink_channel_t chan);
+
 // important note: despite the names, these messages do NOT check to
 // see if the payload will fit in the buffer.  They check to see if
 // the packed message along with any channel overhead will fit.
@@ -40,15 +42,16 @@
 
 // HAVE_PAYLOAD_SPACE evaluates to an expression that can be used
 // anywhere in the code to determine if the mavlink message with ID id
-// can currently fit in the output of _chan.
-#define HAVE_PAYLOAD_SPACE(chan, id) (comm_get_txspace(chan) >= PAYLOAD_SIZE(chan, id))
+// can currently fit in the output of _chan.  Note the use of the ","
+// operator here to increment a counter.
+#define HAVE_PAYLOAD_SPACE(_chan, id) (comm_get_txspace(_chan) >= PAYLOAD_SIZE(_chan, id) ? (gcs_out_of_space_to_send_count(_chan), true) : false)
 
 // CHECK_PAYLOAD_SIZE - macro which may only be used within a
 // GCS_MAVLink object's methods.  It inserts code which will
 // immediately return false from the current function if there is no
 // room to fit the mavlink message with id id on the current object's
 // output
-#define CHECK_PAYLOAD_SIZE(id) if (txspace() < unsigned(packet_overhead()+MAVLINK_MSG_ID_ ## id ## _LEN)) return false
+#define CHECK_PAYLOAD_SIZE(id) if (txspace() < unsigned(packet_overhead()+MAVLINK_MSG_ID_ ## id ## _LEN)) { gcs_out_of_space_to_send_count(chan); return false; }
 
 // CHECK_PAYLOAD_SIZE2 - macro which inserts code which will
 // immediately return false from the current function if there is no
@@ -115,6 +118,9 @@ public:
         return MIN(_port->txspace(), 8192U);
     }
 
+    // this is called when we discover we'd like to send something but can't:
+    void out_of_space_to_send() { out_of_space_to_send_count++; }
+
     void send_mission_ack(const mavlink_message_t &msg,
                           MAV_MISSION_TYPE mission_type,
                           MAV_MISSION_RESULT result) const {
@@ -804,6 +810,7 @@ private:
 
     uint8_t last_tx_seq;
     uint16_t send_packet_count;
+    uint16_t out_of_space_to_send_count; // number of times HAVE_PAYLOAD_SPACE and friends have returned false
 
 #if GCS_DEBUG_SEND_MESSAGE_TIMINGS
     struct {
diff --git a/libraries/GCS_MAVLink/GCS_Common.cpp b/libraries/GCS_MAVLink/GCS_Common.cpp
index 795ab4ab31..24b4886b86 100644
--- a/libraries/GCS_MAVLink/GCS_Common.cpp
+++ b/libraries/GCS_MAVLink/GCS_Common.cpp
@@ -1512,7 +1512,7 @@ void GCS_MAVLINK::log_mavlink_stats()
         flags |= (uint8_t)Flags::LOCKED;
     }
 
-    const struct log_MAV pkt = {
+    const struct log_MAV pkt{
     LOG_PACKET_HEADER_INIT(LOG_MAV_MSG),
     time_us                : AP_HAL::micros64(),
     chan                   : (uint8_t)chan,
@@ -1520,7 +1520,8 @@ void GCS_MAVLINK::log_mavlink_stats()
     packet_rx_success_count: status->packet_rx_success_count,
     packet_rx_drop_count   : status->packet_rx_drop_count,
     flags                  : flags,
-    stream_slowdown_ms     : stream_slowdown_ms
+    stream_slowdown_ms     : stream_slowdown_ms,
+    times_full             : out_of_space_to_send_count,
     };
 
     AP::logger().WriteBlock(&pkt, sizeof(pkt));